General chemistry: textbook / A. V. Zholnin; ed. V. A. Popkova, A. V. Zholnina. - 2012. - 400 p.: ill.

Chapter 7. COMPLEX COMPOUNDS

Chapter 7. COMPLEX COMPOUNDS

The complexing elements are the organizers of life.

K. B. Yatsimirsky

Complex compounds are the most extensive and diverse class of compounds. Living organisms contain complex compounds of biogenic metals with proteins, amino acids, porphyrins, nucleic acids, carbohydrates, and macrocyclic compounds. The most important processes of vital activity proceed with the participation of complex compounds. Some of them (hemoglobin, chlorophyll, hemocyanin, vitamin B 12, etc.) play a significant role in biochemical processes. Many drugs contain metal complexes. For example, insulin (zinc complex), vitamin B 12 (cobalt complex), platinol (platinum complex), etc.

7.1. COORDINATION THEORY OF A. WERNER

The structure of complex compounds

During the interaction of particles, mutual coordination of particles is observed, which can be defined as the process of complex formation. For example, the process of hydration of ions ends with the formation of aqua complexes. Complex formation reactions are accompanied by the transfer of electron pairs and lead to the formation or destruction of higher-order compounds, the so-called complex (coordination) compounds. A feature of complex compounds is the presence in them of a coordination bond that arose according to the donor-acceptor mechanism:

Complex compounds are compounds that exist both in the crystalline state and in solution.

which is the presence of a central atom surrounded by ligands. Complex compounds can be considered as complex compounds of a higher order, consisting of simple molecules capable of independent existence in solution.

According to Werner's coordination theory, in a complex compound, internal And outer sphere. The central atom with its surrounding ligands form the inner sphere of the complex. It is usually enclosed in square brackets. Everything else in a complex compound is the outer sphere and is written in square brackets. A certain number of ligands is placed around the central atom, which is determined coordination number(kch). The number of coordinated ligands is most often 6 or 4. The ligand occupies a coordination site near the central atom. Coordination changes the properties of both the ligands and the central atom. Often, coordinated ligands cannot be detected using chemical reactions characteristic of them in the free state. More tightly bound particles of the inner sphere are called complex (complex ion). Attraction forces act between the central atom and ligands (a covalent bond is formed according to the exchange and (or) donor-acceptor mechanism), and repulsive forces act between ligands. If the charge of the inner sphere is 0, then there is no outer coordination sphere.

Central atom (complexing agent)- an atom or ion that occupies a central position in a complex compound. The role of a complexing agent is most often performed by particles that have free orbits and a sufficiently large positive nuclear charge, and therefore can be electron acceptors. These are cations of transition elements. The strongest complexing agents are elements of groups IB and VIIIB. Rarely as a complex

neutral atoms of d-elements and non-metal atoms in various degrees of oxidation - . The number of free atomic orbitals provided by the complexing agent determines its coordination number. The value of the coordination number depends on many factors, but usually it is equal to twice the charge of the complexing ion:

Ligands- ions or molecules that are directly associated with the complexing agent and are donors of electron pairs. These electron-rich systems, which have free and mobile electron pairs, can be electron donors, for example:

Compounds of p-elements exhibit complexing properties and act as ligands in a complex compound. Ligands can be atoms and molecules (protein, amino acids, nucleic acids, carbohydrates). According to the number of bonds formed by ligands with the complexing agent, ligands are divided into mono-, di-, and polydentate ligands. The above ligands (molecules and anions) are monodentate, since they are donors of one electron pair. Bidentate ligands include molecules or ions containing two functional groups capable of being a donor of two electron pairs:

Polydentate ligands include the 6-dentate ligand of ethylenediaminetetraacetic acid:

The number of places occupied by each ligand in the inner sphere of the complex compound is called coordination capacity (denticity) of the ligand. It is determined by the number of electron pairs of the ligand that participate in the formation of a coordination bond with the central atom.

In addition to complex compounds, coordination chemistry covers double salts, crystalline hydrates, which decompose in an aqueous solution into constituent parts, which in the solid state in many cases are constructed similarly to complex ones, but are unstable.

The most stable and diverse complexes in terms of composition and the functions they perform form d-elements. Of particular importance are complex compounds of transition elements: iron, manganese, titanium, cobalt, copper, zinc and molybdenum. Biogenic s-elements (Na, K, Mg, Ca) form complex compounds only with ligands of a certain cyclic structure, also acting as a complexing agent. Main part R-elements (N, P, S, O) is the active active part of complexing particles (ligands), including bioligands. This is their biological significance.

Therefore, the ability to complex formation is a common property of the chemical elements of the periodic system, this ability decreases in the following order: f> d> p> s.

7.2. DETERMINATION OF THE CHARGE OF MAIN PARTICLES OF A COMPLEX COMPOUND

The charge of the inner sphere of a complex compound is the algebraic sum of the charges of its constituent particles. For example, the magnitude and sign of the charge of a complex are determined as follows. The charge of the aluminum ion is +3, the total charge of the six hydroxide ions is -6. Therefore, the charge of the complex is (+3) + (-6) = -3 and the formula of the complex is 3- . The charge of the complex ion is numerically equal to the total charge of the outer sphere and is opposite in sign to it. For example, the charge of the outer sphere K 3 is +3. Therefore, the charge of the complex ion is -3. The charge of the complexing agent is equal in magnitude and opposite in sign to the algebraic sum of the charges of all other particles of the complex compound. Hence, in K 3 the charge of the iron ion is +3, since the total charge of all other particles of the complex compound is (+3) + (-6) = -3.

7.3. NOMENCLATURE OF COMPLEX COMPOUNDS

The basics of nomenclature are developed in the classic works of Werner. In accordance with them, in a complex compound, the cation is first called, and then the anion. If the compound is of a non-electrolyte type, then it is called in one word. The name of the complex ion is written in one word.

The neutral ligand is named the same as the molecule, and an "o" is added to the anion ligands. For a coordinated water molecule, the designation "aqua-" is used. To indicate the number of identical ligands in the inner sphere of the complex, the Greek numerals di-, tri-, tetra-, penta-, hexa-, etc. are used as a prefix before the name of the ligands. The prefix monone is used. The ligands are listed in alphabetical order. The name of the ligand is considered as a single entity. After the name of the ligand, the name of the central atom follows, indicating the degree of oxidation, which is indicated by Roman numerals in parentheses. The word ammine (with two "m") is written in relation to ammonia. For all other amines, only one "m" is used.

C1 3 - hexamminecobalt (III) chloride.

C1 3 - aquapentamminecobalt (III) chloride.

Cl 2 - pentamethylamminechlorocobalt (III) chloride.

Diamminedibromoplatinum (II).

If the complex ion is an anion, then its Latin name has the ending "am".

(NH 4) 2 - ammonium tetrachloropalladate (II).

K - potassium pentabromoammineplatinate (IV).

K 2 - potassium tetrarodanocobaltate (II).

The name of a complex ligand is usually enclosed in parentheses.

NO 3 - dichloro-di-(ethylenediamine) cobalt (III) nitrate.

Br - bromo-tris-(triphenylphosphine) platinum (II) bromide.

In cases where the ligand binds two central ions, the Greek letter is used before its nameμ.

Such ligands are called bridge and listed last.

7.4. CHEMICAL BOND AND STRUCTURE OF COMPLEX COMPOUNDS

The donor-acceptor interactions between the ligand and the central atom play an important role in the formation of complex compounds. The electron pair donor is usually a ligand. An acceptor is a central atom that has free orbitals. This bond is strong and does not break when the complex is dissolved (nonionogenic), and it is called coordination.

Along with o-bonds, π-bonds are formed by the donor-acceptor mechanism. In this case, the metal ion serves as a donor, donating its paired d-electrons to the ligand, which has energetically favorable vacant orbitals. Such relationships are called dative. They are formed:

a) due to the overlap of the vacant p-orbitals of the metal with the d-orbital of the metal, on which there are electrons that have not entered into a σ-bond;

b) when the vacant d-orbitals of the ligand overlap with the filled d-orbitals of the metal.

A measure of its strength is the degree of overlap between the orbitals of the ligand and the central atom. The orientation of the bonds of the central atom determines the geometry of the complex. To explain the direction of bonds, the concept of hybridization of atomic orbitals of the central atom is used. Hybrid orbitals of the central atom are the result of mixing unequal atomic orbitals, as a result, the shape and energy of the orbitals change mutually, and orbitals of a new identical shape and energy are formed. The number of hybrid orbitals is always equal to the number of original ones. Hybrid clouds are located in the atom at the maximum distance from each other (Table 7.1).

Table 7.1. Types of hybridization of atomic orbitals of a complexing agent and the geometry of some complex compounds

The spatial structure of the complex is determined by the type of hybridization of valence orbitals and the number of unshared electron pairs contained in its valence energy level.

The efficiency of the donor-acceptor interaction between the ligand and the complexing agent, and, consequently, the strength of the bond between them (the stability of the complex) is determined by their polarizability, i.e. the ability to transform their electron shells under external influence. On this basis, the reagents are divided into "hard" or low polarizable, and "soft" - easily polarizable. The polarity of an atom, molecule or ion depends on their size and the number of electron layers. The smaller the radius and electrons of a particle, the less polarized it is. The smaller the radius and the fewer electrons a particle has, the worse it polarizes.

Hard acids form strong (hard) complexes with electronegative O, N, F atoms of ligands (hard bases), while soft acids form strong (soft) complexes with donor P, S, and I atoms of ligands having low electronegativity and high polarizability. We observe here the manifestation of the general principle "like with like".

Due to their rigidity, sodium and potassium ions practically do not form stable complexes with biosubstrates and are found in physiological media in the form of aquacomplexes. Ions Ca 2 + and Mg 2 + form quite stable complexes with proteins and therefore in physiological media are in both ionic and bound states.

Ions of d-elements form strong complexes with biosubstrates (proteins). And soft acids Cd, Pb, Hg are highly toxic. They form strong complexes with proteins containing R-SH sulfhydryl groups:

The cyanide ion is toxic. The soft ligand actively interacts with d-metals in complexes with biosubstrates, activating the latter.

7.5. DISSOCIATION OF COMPLEX COMPOUNDS. STABILITY OF COMPLEXES. LABILE AND INERT COMPLEXES

When complex compounds are dissolved in water, they usually decompose into ions of the outer and inner spheres, like strong electrolytes, since these ions are bound ionogenically, mainly by electrostatic forces. This is estimated as the primary dissociation of complex compounds.

The secondary dissociation of a complex compound is the disintegration of the inner sphere into its constituent components. This process proceeds according to the type of weak electrolytes, since the particles of the inner sphere are connected nonionically (covalently). Dissociation has a stepwise character:

For a qualitative characteristic of the stability of the inner sphere of a complex compound, an equilibrium constant is used that describes its complete dissociation, called complex instability constant(Kn). For a complex anion, the expression for the instability constant has the form:

The smaller the value of Kn, the more stable is the inner sphere of the complex compound, i.e. the less it dissociates in aqueous solution. Recently, instead of Kn, the value of the stability constant (Ku) is used - the reciprocal of Kn. The larger the Ku value, the more stable the complex.

The stability constants make it possible to predict the direction of ligand exchange processes.

In an aqueous solution, the metal ion exists in the form of aqua complexes: 2+ - hexaaqua iron (II), 2 + - tetraaqua copper (II). When writing formulas for hydrated ions, the coordinated water molecules of the hydration shell are not indicated, but implied. The formation of a complex between a metal ion and some ligand is considered as a reaction of substitution of a water molecule in the inner coordination sphere by this ligand.

Ligand exchange reactions proceed according to the mechanism of S N -type reactions. For example:

The values ​​of the stability constants given in Table 7.2 indicate that due to the complex formation process, strong binding of ions in aqueous solutions occurs, which indicates the effectiveness of using this type of reaction for binding ions, especially with polydentate ligands.

Table 7.2. Stability of zirconium complexes

Unlike ion exchange reactions, the formation of complex compounds is often not a quasi-instantaneous process. For example, when iron (III) reacts with nitrile trimethylenephosphonic acid, the equilibrium is established after 4 days. For the kinetic characteristics of complexes, the concepts are used - labile(fast reacting) and inert(slowly reacting). According to the suggestion of G. Taube, labile complexes are considered to be those that completely exchange ligands for 1 min at room temperature and a solution concentration of 0.1 M. It is necessary to clearly distinguish between thermodynamic concepts [strong (stable) / fragile (unstable)] and kinetic [ inert and labile] complexes.

In labile complexes, ligand substitution occurs rapidly and equilibrium is quickly established. In inert complexes, ligand substitution proceeds slowly.

So, the inert complex 2 + in an acidic environment is thermodynamically unstable: the instability constant is 10 -6, and the labile complex 2- is very stable: the stability constant is 10 -30. Taube associates the lability of complexes with the electronic structure of the central atom. The inertness of complexes is characteristic mainly of ions with an incomplete d-shell. Inert complexes include Co, Cr. Cyanide complexes of many cations with an external level of s 2 p 6 are labile.

7.6. CHEMICAL PROPERTIES OF COMPLEXES

The processes of complex formation affect practically the properties of all particles forming the complex. The higher the strength of the bonds between the ligand and the complexing agent, the less the properties of the central atom and ligands manifest themselves in the solution, and the more pronounced the features of the complex.

Complex compounds exhibit chemical and biological activity as a result of the coordination unsaturation of the central atom (there are free orbitals) and the presence of free electron pairs of ligands. In this case, the complex has electrophilic and nucleophilic properties that differ from those of the central atom and ligands.

It is necessary to take into account the influence on the chemical and biological activity of the structure of the hydration shell of the complex. The process of education

The reduction of complexes affects the acid-base properties of the complex compound. The formation of complex acids is accompanied by an increase in the strength of the acid or base, respectively. So, when complex acids are formed from simple ones, the binding energy with H + ions decreases and the strength of the acid increases accordingly. If there is an OH - ion in the outer sphere, then the bond between the complex cation and the hydroxide ion of the outer sphere decreases, and the basic properties of the complex increase. For example, copper hydroxide Cu (OH) 2 is a weak, sparingly soluble base. Under the action of ammonia on it, copper ammonia (OH) 2 is formed. The charge density of 2 + decreases compared to Cu 2 +, the bond with OH - ions is weakened, and (OH) 2 behaves like a strong base. The acid-base properties of the ligands associated with the complexing agent are usually more pronounced than the acid-base properties of them in the free state. For example, hemoglobin (Hb) or oxyhemoglobin (HbO 2) exhibit acidic properties due to the free carboxyl groups of the globin protein, which is a ligand of HHb ↔ H + + Hb - . At the same time, the hemoglobin anion, due to the amino groups of the globin protein, exhibits basic properties and therefore binds the acidic CO 2 oxide to form the carbaminohemoglobin anion (HbCO 2 -): CO 2 + Hb - ↔ HbCO 2 - .

The complexes exhibit redox properties due to redox transformations of the complexing agent, which forms stable oxidation states. The process of complexation strongly affects the values ​​of the reduction potentials of d-elements. If the reduced form of the cations forms a more stable complex with the given ligand than its oxidized form, then the value of the potential increases. A decrease in the potential value occurs when the oxidized form forms a more stable complex. For example, under the action of oxidizing agents: nitrites, nitrates, NO 2 , H 2 O 2, hemoglobin is converted into methemoglobin as a result of oxidation of the central atom.

The sixth orbital is used in the formation of oxyhemoglobin. The same orbital is involved in the formation of a bond with carbon monoxide. As a result, a macrocyclic complex with iron is formed - carboxyhemoglobin. This complex is 200 times more stable than the iron-oxygen complex in heme.

Rice. 7.1. Chemical transformations of hemoglobin in the human body. Scheme from the book: Slesarev V.I. Fundamentals of Living Chemistry, 2000

The formation of complex ions affects the catalytic activity of complexing ions. In some cases, activity is increasing. This is due to the formation in solution of large structural systems that can participate in the creation of intermediate products and a decrease in the activation energy of the reaction. For example, if Cu 2+ or NH 3 is added to H 2 O 2, the decomposition process is not accelerated. In the presence of the 2+ complex, which is formed in an alkaline medium, the decomposition of hydrogen peroxide is accelerated by 40 million times.

So, on hemoglobin, one can consider the properties of complex compounds: acid-base, complex formation and redox.

7.7. CLASSIFICATION OF COMPLEX COMPOUNDS

There are several classification systems for complex compounds based on different principles.

1. According to the belonging of a complex compound to a certain class of compounds:

Complex acids H 2 ;

Complex bases OH;

Complex salts K 4 .

2. By the nature of the ligand: aqua complexes, ammoniates, acido complexes (anions of various acids, K 4, act as ligands; hydroxo complexes (hydroxyl groups, K 3, as ligands); complexes with macrocyclic ligands, inside which central atom.

3. By the sign of the charge of the complex: cationic - complex cation in the complex compound Cl 3; anionic - a complex anion in a complex compound K; neutral - the charge of the complex is 0. The complex compound of the outer sphere does not have, for example, . This is the formula for an anticancer drug.

4. According to the internal structure of the complex:

a) depending on the number of atoms of the complexing agent: mononuclear- the composition of the complex particle includes one atom of the complexing agent, for example Cl 3 ; multi-core- in the composition of the complex particle there are several atoms of the complexing agent - an iron-protein complex:

b) depending on the number of types of ligands, complexes are distinguished: homogeneous (single-ligand), containing one type of ligand, for example 2+, and heterogeneous (multi-ligand)- two kinds of ligands or more, for example Pt(NH 3) 2 Cl 2 . The complex includes NH 3 and Cl - ligands. For complex compounds containing different ligands in the inner sphere, geometric isomerism is characteristic, when, with the same composition of the inner sphere, the ligands in it are located differently relative to each other.

Geometric isomers of complex compounds differ not only in physical and chemical properties, but also in biological activity. The cis-isomer of Pt(NH 3) 2 Cl 2 has a pronounced antitumor activity, while the trans-isomer does not;

c) depending on the denticity of the ligands forming mononuclear complexes, the following groups can be distinguished:

Mononuclear complexes with monodentate ligands, for example 3+ ;

Mononuclear complexes with polydentate ligands. Complex compounds with polydentate ligands are called chelating compounds;

d) cyclic and acyclic forms of complex compounds.

7.8. CHELATE COMPLEXES. COMPLEXSONS. COMPLEXONATES

Cyclic structures that are formed as a result of the addition of a metal ion to two or more donor atoms belonging to one chelating agent molecule are called chelate compounds. For example, copper glycinate:

In them, the complexing agent, as it were, leads inside the ligand, is covered by bonds, like claws, therefore, other things being equal, they are more stable than compounds that do not contain cycles. The most stable are cycles consisting of five or six links. This rule was first formulated by L.A. Chugaev. Difference

stability of the chelate complex and the stability of its non-cyclic analogue are called chelate effect.

Polydentate ligands that contain 2 types of groups act as a chelating agent:

1) groups capable of forming covalent polar bonds due to exchange reactions (proton donors, electron pair acceptors) -CH 2 COOH, -CH 2 PO (OH) 2, -CH 2 SO 2 OH, - acid groups (centers);

2) electron pair donor groups: ≡N, >NH, >C=O, -S-, -OH, - main groups (centers).

If such ligands saturate the inner coordination sphere of the complex and completely neutralize the charge of the metal ion, then the compounds are called intracomplex. For example, copper glycinate. There is no outer sphere in this complex.

A large group of organic substances containing basic and acid centers in the molecule is called complexones. These are polybasic acids. Chelate compounds formed by complexones when interacting with metal ions are called complexonates, for example, magnesium complexonate with ethylenediaminetetraacetic acid:

In aqueous solution, the complex exists in the anionic form.

Complexons and complexonates are a simple model of more complex compounds of living organisms: amino acids, polypeptides, proteins, nucleic acids, enzymes, vitamins and many other endogenous compounds.

Currently, a huge range of synthetic complexones with various functional groups is being produced. The formulas of the main complexones are presented below:

Complexons, under certain conditions, can provide unshared electron pairs (several) for the formation of a coordination bond with a metal ion (s-, p- or d-element). As a result, stable chelate-type compounds with 4-, 5-, 6-, or 8-membered rings are formed. The reaction proceeds over a wide pH range. Depending on pH, the nature of the complexing agent, its ratio with the ligand, complexonates of various strengths and solubility are formed. The chemistry of the formation of complexonates can be represented by equations using the sodium salt of EDTA (Na 2 H 2 Y) as an example, which dissociates in an aqueous solution: Na 2 H 2 Y→ 2Na + + H 2 Y 2-, and the H 2 Y 2- ion interacts with ions metals, regardless of the degree of oxidation of the metal cation, most often one metal ion (1:1) interacts with one complexone molecule. The reaction proceeds quantitatively (Kp>10 9).

Complexones and complexonates exhibit amphoteric properties in a wide pH range, the ability to participate in oxidation-reduction reactions, complex formation, form compounds with various properties depending on the degree of oxidation of the metal, its coordination saturation, and have electrophilic and nucleophilic properties. All this determines the ability to bind a huge number of particles, which allows a small amount of reagent to solve large and diverse problems.

Another indisputable advantage of complexones and complexonates is their low toxicity and the ability to convert toxic particles

into low-toxic or even biologically active ones. Decomposition products of complexonates do not accumulate in the body and are harmless. The third feature of complexonates is the possibility of their use as a source of trace elements.

Increased digestibility is due to the fact that the trace element is introduced in a biologically active form and has a high membrane permeability.

7.9. PHOSPHORUS-CONTAINING METAL COMPLEXONATES - AN EFFECTIVE FORM OF TRANSFORMATION OF MICRO AND MACRO ELEMENTS INTO A BIOLOGICALLY ACTIVE STATE AND A MODEL FOR STUDYING THE BIOLOGICAL ACTION OF CHEMICAL ELEMENTS

concept biological activity covers a wide range of phenomena. From the point of view of chemical action, biologically active substances (BAS) are commonly understood as substances that can act on biological systems, regulating their vital activity.

The ability to such an impact is interpreted as the ability to exhibit biological activity. Regulation can manifest itself in the effects of stimulation, oppression, development of certain effects. The extreme manifestation of biological activity is biocidal action, when, as a result of the action of a biocide substance on the body, the latter dies. At lower concentrations, in most cases, biocides have a stimulating rather than lethal effect on living organisms.

A large number of such substances are currently known. Nevertheless, in many cases, the use of known biologically active substances is used insufficiently, often with efficiency far from maximum, and the use often leads to side effects that can be eliminated by introducing modifiers into biologically active substances.

Phosphorus-containing complexonates form compounds with various properties depending on the nature, degree of oxidation of the metal, coordination saturation, composition and structure of the hydrate shell. All this determines the multifunctionality of complexonates, their unique ability of substoichiometric action,

the effect of a common ion and provides wide application in medicine, biology, ecology and in various sectors of the national economy.

When the metal ion coordinates the complexon, the electron density is redistributed. Due to the participation of a lone electron pair in the donor-acceptor interaction, the electron density of the ligand (complexon) shifts to the central atom. A decrease in the relatively negative charge on the ligand contributes to a decrease in the Coulomb repulsion of the reagents. Therefore, the coordinated ligand becomes more accessible to attack by a nucleophilic reagent that has an excess of electron density on the reaction center. The shift of the electron density from the complexing agent to the metal ion leads to a relative increase in the positive charge of the carbon atom, and, consequently, to the facilitation of its attack by the nucleophilic reagent, the hydroxyl ion. Among the enzymes that catalyze metabolic processes in biological systems, the hydroxylated complex occupies one of the central places in the mechanism of enzymatic action and detoxification of the body. As a result of the multipoint interaction of the enzyme with the substrate, orientation occurs, which ensures the convergence of active groups in the active center and the transfer of the reaction to the intramolecular regime, before the reaction begins and the transition state is formed, which ensures the enzymatic function of FCM. Conformational changes can occur in enzyme molecules. Coordination creates additional conditions for the redox interaction between the central ion and the ligand, since a direct bond is established between the oxidizing agent and the reducing agent, which ensures the transfer of electrons. FCM transition metal complexes can be characterized by L-M, M-L, M-L-M type electron transitions, in which the orbitals of both the metal (M) and ligands (L) participate, which are respectively linked in the complex by donor-acceptor bonds. Complexons can serve as a bridge along which the electrons of multinuclear complexes oscillate between the central atoms of one or different elements in different oxidation states. (electron and proton transport complexes). Complexons determine the reducing properties of metal complexonates, which allows them to exhibit high antioxidant, adaptogenic properties, homeostatic functions.

So, complexones convert microelements into a biologically active, accessible form for the body. They form stable

more coordinatively saturated particles, incapable of destroying biocomplexes, and, consequently, low-toxic forms. Complexonates favorably act in violation of the microelement homeostasis of the body. Ions of transition elements in the complexonate form act in the body as a factor that determines the high sensitivity of cells to microelements through their participation in the creation of a high concentration gradient, the membrane potential. Transition metal complexonates FKM have bioregulatory properties.

The presence of acidic and basic centers in the composition of FCM provides amphoteric properties and their participation in maintaining acid-base balance (isohydric state).

With an increase in the number of phosphonic groups in the composition of the complexone, the composition and conditions for the formation of soluble and poorly soluble complexes change. An increase in the number of phosphonic groups favors the formation of sparingly soluble complexes in a wider pH range and shifts the area of ​​their existence to the acidic area. The decomposition of the complexes occurs at a pH of more than 9.

The study of the processes of complex formation with complexones made it possible to develop methods for the synthesis of bioregulators:

Growth stimulants of prolonged action in a colloid-chemical form are polynuclear homo- and heterocomplex compounds of titanium and iron;

Growth stimulants in water-soluble form. These are mixed-ligand titanium complexonates based on complexones and an inorganic ligand;

Growth inhibitors - phosphorus-containing complexonates of s-elements.

The biological effect of the synthesized preparations on growth and development was studied in a chronic experiment on plants, animals and humans.

Bioregulation- this is a new scientific direction that allows you to regulate the direction and intensity of biochemical processes, which can be widely used in medicine, animal husbandry and crop production. It is associated with the development of ways to restore the physiological function of the body in order to prevent and treat diseases and age-related pathologies. Complexones and complex compounds based on them can be classified as promising biologically active compounds. The study of their biological action in a chronic experiment showed that chemistry gave into the hands of physicians,

livestock breeders, agronomists and biologists, a new promising tool that allows you to actively influence a living cell, regulate nutritional conditions, growth and development of living organisms.

A study of the toxicity of the complexones and complexonates used showed the complete absence of the effect of drugs on the hematopoietic organs, blood pressure, excitability, respiratory rate: no change in liver function was noted, no toxicological effect on the morphology of tissues and organs was detected. Potassium salt of HEDP has no toxicity at a dose 5-10 times higher than the therapeutic one (10-20 mg/kg) in the study for 181 days. Therefore, complexones are classified as low-toxic compounds. They are used as medicines to combat viral diseases, poisoning with heavy metals and radioactive elements, calcium metabolism disorders, endemic diseases and microelement imbalance in the body. Phosphorus-containing complexons and complexonates do not undergo photolysis.

Progressive pollution of the environment with heavy metals - products of human economic activity is a permanent environmental factor. They can accumulate in the body. Excess and lack of them cause intoxication of the body.

Metal complexonates retain the chelating effect on the ligand (complexone) in the body and are indispensable for maintaining metal ligand homeostasis. Incorporated heavy metals are neutralized to a certain extent in the body, and low resorption capacity prevents the transfer of metals along trophic chains, as a result, this leads to a certain “biominization” of their toxic effect, which is especially important for the Ural region. For example, the free lead ion belongs to thiol poisons, and the strong complexonate of lead with ethylenediaminetetraacetic acid is of low toxicity. Therefore, detoxification of plants and animals consists in the use of metal complexonates. It is based on two thermodynamic principles: their ability to form strong bonds with toxic particles, turning them into poorly soluble or stable compounds in an aqueous solution; their inability to destroy endogenous biocomplexes. In this regard, we consider an important direction in the fight against eco-poisoning and obtaining environmentally friendly products - this is complex therapy of plants and animals.

A study was made of the effect of plant treatment with complexonates of various metals under intensive cultivation technology.

potatoes on the microelement composition of potato tubers. Tuber samples contained 105-116 mg/kg iron, 16-20 mg/kg manganese, 13-18 mg/kg copper and 11-15 mg/kg zinc. The ratio and content of microelements are typical for plant tissues. Tubers grown with and without the use of metal complexonates have almost the same elemental composition. The use of chelates does not create conditions for the accumulation of heavy metals in tubers. Complexonates, to a lesser extent than metal ions, are sorbed by the soil, are resistant to its microbiological effects, which allows them to be retained in the soil solution for a long time. The aftereffect is 3-4 years. They combine well with various pesticides. The metal in the complex has a lower toxicity. Phosphorus-containing metal complexonates do not irritate the mucous membrane of the eyes and do not damage the skin. Sensitizing properties have not been identified, the cumulative properties of titanium complexonates are not pronounced, and in some cases they are very weakly expressed. The cumulation coefficient is 0.9-3.0, which indicates a low potential danger of chronic drug poisoning.

Phosphorus-containing complexes are based on the phosphorus-carbon bond (C-P), which is also found in biological systems. It is part of the phosphonolipids, phosphonoglycans and phosphoproteins of cell membranes. Lipids containing aminophosphonic compounds are resistant to enzymatic hydrolysis, provide stability and, consequently, normal functioning of the outer cell membranes. Synthetic analogues of pyrophosphates - diphosphonates (Р-С-Р) or (Р-С-С-Р) in large doses disrupt calcium metabolism, and in small doses normalize it. Diphosphonates are effective in hyperlipemia and promising from the standpoint of pharmacology.

Diphosphonates containing P-C-P bonds are structural elements of biosystems. They are biologically effective and are analogues of pyrophosphates. Diphosphonates have been shown to be effective in the treatment of various diseases. Diphosphonates are active inhibitors of bone mineralization and resorption. Complexons convert microelements into a biologically active, accessible form for the body, form stable, more coordinatively saturated particles that are unable to destroy biocomplexes, and therefore, low-toxic forms. They determine the high sensitivity of cells to trace elements, participating in the formation of a high concentration gradient. Able to participate in the formation of polynuclear titanium compounds

of a different type - electron and proton transport complexes, participate in the bioregulation of metabolic processes, body resistance, the ability to form bonds with toxic particles, turning them into poorly soluble or soluble, stable, non-destructive endogenous complexes. Therefore, their use for detoxification, elimination from the body, obtaining environmentally friendly products (complex therapy), as well as in industry for the regeneration and disposal of industrial wastes of inorganic acids and transition metal salts is very promising.

7.10. LIGAND EXCHANGE AND METAL EXCHANGE

BALANCE. CHELATHERAPY

If there are several ligands with one metal ion or several metal ions with one ligand capable of forming complex compounds in the system, then competing processes are observed: in the first case, ligand-exchange equilibrium is competition between ligands for a metal ion, in the second case, metal-exchange equilibrium is competition between ions metal for the ligand. The process of formation of the most durable complex will prevail. For example, in solution there are ions: magnesium, zinc, iron (III), copper, chromium (II), iron (II) and manganese (II). When a small amount of ethylenediaminetetraacetic acid (EDTA) is introduced into this solution, competition between metal ions and binding to the iron (III) complex occur, since it forms the most stable complex with EDTA.

Interaction of biometals (Mb) and bioligands (Lb), formation and destruction of vital biocomplexes (MbLb) are constantly taking place in the body:

In the body of humans, animals and plants, there are various mechanisms for protecting and maintaining this balance from various xenobiotics (foreign substances), including heavy metal ions. Ions of heavy metals that are not bound into a complex and their hydroxo complexes are toxic particles (Mt). In these cases, along with the natural metal ligand equilibrium, a new equilibrium may arise, with the formation of more stable foreign complexes containing toxicant metals (MtLb) or toxicant ligands (MbLt), which do not fulfill

essential biological functions. When exogenous toxic particles enter the body, combined equilibria arise and, as a result, competition of processes occurs. The predominant process will be the one that leads to the formation of the most stable complex compound:

Violations of metal ligand homeostasis cause metabolic disorders, inhibit the activity of enzymes, destroy important metabolites such as ATP, cell membranes, and disrupt the ion concentration gradient in cells. Therefore, artificial protection systems are being created. Chelation therapy (complex therapy) takes its due place in this method.

Chelation therapy is the removal of toxic particles from the body, based on their chelation with s-element complexonates. Drugs used to remove toxic particles incorporated in the body are called detoxifiers.(Lg). Chelation of toxic species with metal complexonates (Lg) converts toxic metal ions (Mt) into non-toxic (MtLg) bound forms suitable for isolation and membrane penetration, transport and excretion from the body. They retain a chelating effect in the body both for the ligand (complexon) and for the metal ion. This ensures the metal ligand homeostasis of the body. Therefore, the use of complexonates in medicine, animal husbandry, and crop production provides detoxification of the body.

The basic thermodynamic principles of chelation therapy can be formulated in two positions.

I. A detoxicant (Lg) must effectively bind toxicant ions (Mt, Lt), newly formed compounds (MtLg) must be stronger than those that existed in the body:

II. The detoxifier should not destroy vital complex compounds (MbLb); compounds that can be formed during the interaction of a detoxifier and biometal ions (MbLg) should be less strong than those existing in the body:

7.11. APPLICATION OF COMPLEXONS AND COMPLEXONATES IN MEDICINE

Complexone molecules practically do not undergo splitting or any change in the biological environment, which is their important pharmacological feature. Complexons are insoluble in lipids and highly soluble in water, so they do not penetrate or penetrate poorly through cell membranes, and therefore: 1) are not excreted by the intestines; 2) the absorption of complexing agents occurs only when they are injected (only penicillamine is taken orally); 3) in the body, complexons circulate mainly in the extracellular space; 4) excretion from the body is carried out mainly through the kidneys. This process is fast.

Substances that eliminate the effects of poisons on biological structures and inactivate poisons through chemical reactions are called antidotes.

One of the first antidotes to be used in chelation therapy is British Anti-Lewisite (BAL). Unithiol is currently used:

This drug effectively removes arsenic, mercury, chromium and bismuth from the body. The most widely used for poisoning with zinc, cadmium, lead and mercury are complexones and complexonates. Their use is based on the formation of stronger complexes with metal ions than complexes of the same ions with sulfur-containing groups of proteins, amino acids and carbohydrates. EDTA preparations are used to remove lead. The introduction of large doses of drugs into the body is dangerous, since they bind calcium ions, which leads to disruption of many functions. Therefore, apply tetacin(CaNa 2 EDTA), which is used to remove lead, cadmium, mercury, yttrium, cerium and other rare earth metals and cobalt.

Since the first therapeutic use of tetacin in 1952, this drug has found wide use in the clinic of occupational diseases and continues to be an indispensable antidote. The mechanism of action of tetacin is very interesting. Ions-toxicants displace the coordinated calcium ion from tetacin due to the formation of stronger bonds with oxygen and EDTA. The calcium ion, in turn, displaces the two remaining sodium ions:

Tetacin is introduced into the body in the form of a 5-10% solution, the basis of which is saline. So, already 1.5 hours after intraperitoneal injection, 15% of the administered dose of tetacin remains in the body, after 6 hours - 3%, and after 2 days - only 0.5%. The drug acts effectively and quickly when using the inhalation method of tetacin administration. It is rapidly absorbed and circulates in the blood for a long time. In addition, tetacin is used in protection against gas gangrene. It inhibits the action of zinc and cobalt ions, which are activators of the enzyme lecithinase, which is a gas gangrene toxin.

The binding of toxicants with tetacin into a low-toxic and more durable chelate complex, which is not destroyed and is easily excreted from the body through the kidneys, provides detoxification and balanced mineral nutrition. Close in structure and composition to pre-

paratam EDTA is the sodium-calcium salt of diethylenetriamine-pentaacetic acid (CaNa 3 DTPA) - pentacin and sodium salt of dacid (Na 6 DTPF) - trimefacin. Pentacin is used mainly for poisoning with iron, cadmium and lead compounds, as well as for the removal of radionuclides (technetium, plutonium, uranium).

Sodium salt of ethyacid (СаNa 2 EDTP) phosphicin successfully used to remove mercury, lead, beryllium, manganese, actinides and other metals from the body. Complexonates are very effective in removing some toxic anions. For example, cobalt (II) ethylenediaminetetraacetate, which forms a mixed-ligand complex with CN - , can be recommended as an antidote for cyanide poisoning. A similar principle underlies methods for removing toxic organic substances, including pesticides containing functional groups with donor atoms capable of interacting with the complexonate metal.

An effective drug is succimer(dimercaptosuccinic acid, dimercaptosuccinic acid, chemet). It strongly binds almost all toxicants (Hg, As, Pb, Cd), but removes ions of biogenic elements (Cu, Fe, Zn, Co) from the body, so it is almost never used.

Phosphorus-containing complexonates are powerful inhibitors of crystal formation of phosphates and calcium oxalates. As an anticalcifying drug in the treatment of urolithiasis, ksidifon, a potassium-sodium salt of OEDP, is proposed. Diphosphonates, in addition, in minimal doses increase the incorporation of calcium into bone tissue, and prevent its pathological exit from the bones. HEDP and other diphosphonates prevent various types of osteoporosis, including renal osteodystrophy, periodontal

ny destruction, as well as the destruction of the transplanted bone in animals. The anti-atherosclerotic effect of HEDP has also been described.

In the USA, a number of diphosphonates, in particular HEDP, have been proposed as pharmaceutical preparations for the treatment of humans and animals suffering from metastasized bone cancer. By regulating membrane permeability, bisphosphonates promote the transport of antitumor drugs into the cell, and hence the effective treatment of various oncological diseases.

One of the urgent problems of modern medicine is the task of rapid diagnosis of various diseases. In this aspect, of undoubted interest is a new class of preparations containing cations capable of performing the functions of a probe - radioactive magnetorelaxation and fluorescent labels. Radioisotopes of certain metals are used as the main components of radiopharmaceuticals. Chelation of the cations of these isotopes with complexones makes it possible to increase their toxicological acceptability for the body, to facilitate their transportation, and to ensure, within certain limits, the selectivity of concentration in certain organs.

These examples by no means exhaust the whole variety of forms of application of complexonates in medicine. Thus, the dipotassium salt of magnesium ethylenediaminetetraacetate is used to regulate the fluid content in tissues in pathology. EDTA is used in the composition of anticoagulant suspensions used in the separation of blood plasma, as a stabilizer of adenosine triphosphate in the determination of blood glucose, in the clarification and storage of contact lenses. Diphosphonates are widely used in the treatment of rheumatoid diseases. They are especially effective as anti-arthritic agents in combination with anti-inflammatory agents.

7.12. COMPLEXES WITH MACROCYCLIC COMPOUNDS

Among natural complex compounds, a special place is occupied by macrocomplexes based on cyclic polypeptides containing internal cavities of certain sizes, in which there are several oxygen-containing groups capable of binding cations of those metals, including sodium and potassium, whose dimensions correspond to the dimensions of the cavity. Such substances, being in biological

Rice. 7.2. Complex of valinomycin with K+ ion

ical materials, provide transport of ions through membranes and are therefore called ionophores. For example, valinomycin transports a potassium ion across the membrane (Fig. 7.2).

With the help of another polypeptide - gramicidin A sodium cations are transported by the relay mechanism. This polypeptide is folded into a "tube", the inner surface of which is lined with oxygen-containing groups. The result is

a sufficiently long hydrophilic channel with a certain cross section corresponding to the size of the sodium ion. The sodium ion, entering the hydrophilic channel from one side, is transferred from one to the other oxygen groups, like a relay race through an ion-conducting channel.

Thus, a cyclic polypeptide molecule has an intramolecular cavity, into which a substrate of a certain size and geometry can enter according to the principle of a key and a lock. The cavity of such internal receptors is lined with active centers (endoreceptors). Depending on the nature of the metal ion, non-covalent interaction (electrostatic, hydrogen bonding, van der Waals forces) with alkali metals and covalent interaction with alkaline earth metals can occur. As a result of this, supramolecules- complex associates consisting of two or more particles held together by intermolecular forces.

The most common in living nature are tetradentate macrocycles - porphins and corrinoids close to them in structure. Schematically, the tetradent cycle can be represented in the following form (Fig. 7.3), where the arcs mean the same type of carbon chains connecting donor nitrogen atoms in a closed cycle; R 1 , R 2 , R 3 , P 4 are hydrocarbon radicals; M n+ - metal ion: in chlorophyll Mg 2+ ion, in hemoglobin Fe 2+ ion, in hemocyanin Cu 2+ ion, in vitamin B 12 (cobalamin) Co 3+ ion.

Donor nitrogen atoms are located at the corners of the square (indicated by the dotted line). They are tightly coordinated in space. That's why

porphyrins and corrinoids form strong complexes with cations of various elements and even alkaline earth metals. It is significant that Regardless of the denticity of the ligand, the chemical bond and structure of the complex are determined by donor atoms. For example, copper complexes with NH 3 , ethylenediamine, and porphyrin have the same square structure and a similar electronic configuration. But polydentate ligands bind to metal ions much more strongly than monodentate ligands.

Rice. 7.3. Tetradentate macrocycle

with the same donor atoms. The strength of ethylenediamine complexes is 8-10 orders of magnitude greater than the strength of the same metals with ammonia.

Bioinorganic complexes of metal ions with proteins are called bioclusters - complexes of metal ions with macrocyclic compounds (Fig. 7.4).

Rice. 7.4. Schematic representation of the structure of bioclusters of certain sizes of protein complexes with ions of d-elements. Types of interactions of a protein molecule. M n+ - active center metal ion

There is a cavity inside the biocluster. It includes a metal that interacts with donor atoms of the linking groups: OH - , SH - , COO - , -NH 2 , proteins, amino acids. The most famous metal-

ments (carbonic anhydrase, xanthine oxidase, cytochromes) are bioclusters whose cavities form enzyme centers containing Zn, Mo, Fe, respectively.

7.13. MULTICORE COMPLEXES

Heterovalent and heteronuclear complexes

Complexes, which include several central atoms of one or different elements, are called multi-core. The possibility of forming multinuclear complexes is determined by the ability of some ligands to bind to two or three metal ions. Such ligands are called bridge. Respectively bridge are called complexes. In principle, one-atom bridges are also possible, for example:

They use lone electron pairs belonging to the same atom. The role of bridges can be played polyatomic ligands. In such bridges, unshared electron pairs belonging to different atoms are used. polyatomic ligand.

A.A. Grinberg and F.M. Filinov studied bridging compounds of composition , in which the ligand binds complex compounds of the same metal, but in different oxidation states. G. Taube named them electron transfer complexes. He investigated the reactions of electron transfer between the central atoms of various metals. Systematic studies of the kinetics and mechanism of redox reactions have led to the conclusion that the transfer of an electron between two complexes is

proceeds through the resulting ligand bridge. The exchange of an electron between 2 + and 2 + occurs through the formation of an intermediate bridge complex (Fig. 7.5). Electron transfer occurs through the chloride bridging ligand, ending in the formation of 2+ complexes; 2+.

Rice. 7.5. Electron transfer in an intermediate multinuclear complex

A wide variety of polynuclear complexes has been obtained through the use of organic ligands containing several donor groups. The condition for their formation is such an arrangement of donor groups in the ligand that does not allow chelate cycles to close. It is not uncommon for a ligand to close the chelate cycle and simultaneously act as a bridge.

The active principle of electron transfer are transition metals that exhibit several stable oxidation states. This gives titanium, iron and copper ions ideal electron carrier properties. The set of options for the formation of heterovalent (HVA) and heteronuclear complexes (HNC) based on Ti and Fe is shown in Fig. 3. 7.6.

reaction

Reaction (1) is called cross reaction. In exchange reactions, the intermediate will be heterovalent complexes. All theoretically possible complexes are actually formed in solution under certain conditions, which is proved by various physicochemical studies.

Rice. 7.6. Formation of heterovalent complexes and heteronuclear complexes containing Ti and Fe

methods. For electron transfer to occur, the reactants must be in states close in energy. This requirement is called the Franck-Condon principle. Electron transfer can occur between atoms of the same transition element, which are in different degrees of HWC oxidation, or different HJC elements, the nature of metal centers of which is different. These compounds can be defined as electron transport complexes. They are convenient carriers of electrons and protons in biological systems. The addition and release of an electron causes changes only in the electronic configuration of the metal, without changing the structure of the organic component of the complex. All these elements have several stable oxidation states (Ti +3 and +4; Fe +2 and +3; Cu +1 and +2). In our opinion, these systems are given by nature a unique role of ensuring the reversibility of biochemical processes with minimal energy costs. Reversible reactions include reactions that have thermodynamic and thermochemical constants from 10 -3 to 10 3 and with a small value of ΔG o and E o processes. Under these conditions, the initial substances and reaction products can be in comparable concentrations. When changing them in a certain range, it is easy to achieve the reversibility of the process, therefore, in biological systems, many processes are oscillatory (wave) in nature. Redox systems containing the above pairs cover a wide range of potentials, which allows them to enter into interactions accompanied by moderate changes in Δ Go And E°, with many substrates.

The probability of formation of HVA and HJA increases significantly when the solution contains potentially bridging ligands, i.e. molecules or ions (amino acids, hydroxy acids, complexones, etc.) capable of linking two metal centers at once. The possibility of delocalization of an electron in the HWC contributes to a decrease in the total energy of the complex.

More realistically, the set of possible options for the formation of HWC and HJA, in which the nature of the metal centers is different, is seen in Fig. 7.6. A detailed description of the formation of HVA and HNA and their role in biochemical systems are considered in the works of A.N. Glebova (1997). Redox pairs must structurally adjust to each other, then the transfer becomes possible. By selecting the components of the solution, one can "lengthen" the distance over which an electron is transferred from the reducing agent to the oxidizing agent. With a coordinated movement of particles, an electron can be transferred over long distances by the wave mechanism. As a "corridor" can be a hydrated protein chain, etc. The probability of electron transfer to a distance of up to 100A is high. The length of the "corridor" can be increased by additives (alkali metal ions, supporting electrolytes). This opens up great opportunities in the field of controlling the composition and properties of HWC and HJA. In solutions, they play the role of a kind of "black box" filled with electrons and protons. Depending on the circumstances, he can give them to other components or replenish his "reserves". The reversibility of reactions involving them makes it possible to repeatedly participate in cyclic processes. Electrons move from one metal center to another, oscillate between them. The complex molecule remains asymmetric and can take part in redox processes. HWC and HJAC are actively involved in oscillatory processes in biological media. This type of reaction is called oscillatory reactions. They are found in enzymatic catalysis, protein synthesis and other biochemical processes accompanying biological phenomena. These include periodic processes of cellular metabolism, waves of activity in the heart tissue, in brain tissue, and processes occurring at the level of ecological systems. An important stage of metabolism is the splitting of hydrogen from nutrients. In this case, hydrogen atoms pass into the ionic state, and the electrons separated from them enter the respiratory chain and give up their energy to the formation of ATP. As we have established, titanium complexonates are active carriers of not only electrons, but also protons. The ability of titanium ions to fulfill their role in the active center of enzymes such as catalases, peroxidases and cytochromes is determined by its high ability to complex formation, the formation of coordinated ion geometry, the formation of polynuclear HVA and HJA of various compositions and properties as a function of pH, the concentration of the transition element Ti and the organic component of the complex, their molar ratio. This ability is manifested in an increase in the selectivity of the complex

in relation to substrates, products of metabolic processes, activation of bonds in the complex (enzyme) and substrate through coordination and change in the shape of the substrate in accordance with the steric requirements of the active center.

Electrochemical transformations in the body associated with the transfer of electrons are accompanied by a change in the degree of oxidation of particles and the appearance of a redox potential in solution. A large role in these transformations belongs to the multinuclear HVA and HNA complexes. They are active regulators of free radical processes, a system for the utilization of reactive oxygen species, hydrogen peroxide, oxidizing agents, radicals, and are involved in the oxidation of substrates, as well as in maintaining antioxidant homeostasis, in protecting the body from oxidative stress. Their enzymatic action on biosystems is similar to enzymes (cytochromes, superoxide dismutase, catalase, peroxidase, glutathione reductase, dehydrogenases). All this indicates high antioxidant properties of complexonates of transition elements.

7.14. QUESTIONS AND TASKS FOR SELF-CHECKING OF PREPAREDNESS FOR LESSONS AND EXAMS

1. Give the concept of complex compounds. How do they differ from double salts, and what do they have in common?

2. Make formulas of complex compounds according to their name: ammonium dihydroxotetrachloroplatinate (IV), triammintrinitrocobalt (III), give their characteristics; indicate the internal and external coordination sphere; the central ion and the degree of its oxidation: ligands, their number and denticity; the nature of the connections. Write the dissociation equation in an aqueous solution and the expression for the stability constant.

3. General properties of complex compounds, dissociation, stability of complexes, chemical properties of complexes.

4. How is the reactivity of complexes characterized from thermodynamic and kinetic positions?

5. Which amino complexes will be more durable than tetraamino-copper (II), and which ones will be less durable?

6. Give examples of macrocyclic complexes formed by alkali metal ions; d-element ions.

7. On what basis are complexes classified as chelated? Give examples of chelate and non-chelate complex compounds.

8. Using the example of copper glycinate, give the concept of intracomplex compounds. Write the structural formula of magnesium complexonate with ethylenediaminetetraacetic acid in sodium form.

9. Give a schematic structural fragment of any polynuclear complex.

10. Define polynuclear, heteronuclear and heterovalent complexes. The role of transition metals in their formation. The biological role of these components.

11. What types of chemical bonds are found in complex compounds?

12. List the main types of hybridization of atomic orbitals that can occur at the central atom in the complex. What is the geometry of the complex depending on the type of hybridization?

13. Based on the electronic structure of the atoms of the elements of s-, p- and d-blocks, compare the ability to complex formation and their place in the chemistry of complexes.

14. Define complexones and complexonates. Give examples of the most used in biology and medicine. Give the thermodynamic principles on which chelation therapy is based. The use of complexonates for the neutralization and elimination of xenobiotics from the body.

15. Consider the main cases of violation of metal-ligand homeostasis in the human body.

16. Give examples of biocomplex compounds containing iron, cobalt, zinc.

17. Examples of competing processes involving hemoglobin.

18. The role of metal ions in enzymes.

19. Explain why for cobalt in complexes with complex ligands (polydentate) the oxidation state +3 is more stable, and in ordinary salts, such as halides, sulfates, nitrates, the oxidation state is +2?

20. For copper, oxidation states +1 and +2 are characteristic. Can copper catalyze electron transfer reactions?

21. Can zinc catalyze redox reactions?

22. What is the mechanism of action of mercury as a poison?

23. Indicate the acid and base in the reaction:

AgNO 3 + 2NH 3 \u003d NO 3.

24. Explain why the potassium-sodium salt of hydroxyethylidene diphosphonic acid, and not HEDP, is used as a drug.

25. How is the transport of electrons in the body carried out with the help of metal ions, which are part of biocomplex compounds?

7.15. TESTS

1. The oxidation state of the central atom in the complex ion is 2- is equal to:

a)-4;

b) +2;

at 2;

d) +4.

2. The most stable complex ion:

a) 2-, Kn = 8.5x10 -15;

b) 2-, Kn = 1.5x10 -30;

c) 2-, Kn = 4x10 -42;

d) 2-, Kn = 1x10 -21.

3. The solution contains 0.1 mol of the PtCl 4 4NH 3 compound. Reacting with AgNO 3 , it forms 0.2 mol of AgCl precipitate. Give the starting substance the coordination formula:

a)Cl;

b) Cl 3 ;

c) Cl 2 ;

d) Cl 4 .

4. What is the shape of the complexes formed as a result of sp 3 d 2-gi- breeding?

1) tetrahedron;

2) square;

4) trigonal bipyramid;

5) linear.

5. Choose the formula for the compound pentaamminechlorocobalt (III) sulfate:

a) Na 3 ;

6) [CoCl 2 (NH 3) 4 ]Cl;

c) K 2 [Co(SCN) 4];

d) SO 4 ;

e) [Co(H 2 O) 6 ] C1 3 .

6. What ligands are polydentate?

a) C1 -;

b) H 2 O;

c) ethylenediamine;

d) NH 3 ;

e) SCN - .

7. Complexing agents are:

a) electron pair donor atoms;

c) atoms- and ions-acceptors of electron pairs;

d) atoms- and ions-donors of electron pairs.

8. The elements with the least complexing ability are:

a)s; c) d;

b) p; d) f

9. Ligands are:

a) electron pair donor molecules;

b) ions-acceptors of electron pairs;

c) molecules- and ions-donors of electron pairs;

d) molecules- and ions-acceptors of electron pairs.

10. Communication in the internal coordination sphere of the complex:

a) covalent exchange;

b) covalent donor-acceptor;

c) ionic;

d) hydrogen.

11. The best complexing agent will be:

complex compounds. Their structure is based on the coordination theory of A. Werner. Complex ion, its charge. Cationic, anionic, neutral complexes. Nomenclature, examples.

Ligand substitution reactions. Complex ion instability constant, stability constant.

To instability is the ratio of the products of the concentration of decayed ions to the undecayed amount.

K set \u003d 1 / K nest (reciprocal)

Secondary dissociation - the disintegration of the inner sphere of the complex into its constituent components.

43. Competition for a ligand or for a complexing agent: isolated and combined equilibria of ligand substitution. General constant of combined equilibrium of ligand substitution.

As a result of competition, the proton destroys a sufficiently strong complex, forming a weakly dissociating substance - water.

Cl + NiS0 4 +4NH 3 ^ S0 4 + AgCl I

This is already an example of ligand competition for a complexing agent, with the formation of a more stable complex (K H + \u003d 9.3-1 (G 8; K H [M (W 3) 6 ] 2+ \u003d 1.9-10 -9) and a sparingly soluble compound AgCl - K s \u003d 1.8 10 "10

Ideas about the structure of metalloenzymes and other biocomplex compounds (hemoglobin, cytochromes, cobalamins). Physico-chemical principles of oxygen transport by hemoglobin

Cobalamins. Vitamin B 12 called a group of cobalt-containing biologically active substances called cobalamins. They are actually cyanocobalamin, hydroxycobalamin and two coenzymatic forms of vitamin B 12: methylcobalamin and 5-deoxyadenosylcobalamin.

Sometimes, in a narrower sense, vitamin B 12 is called cyanocobalamin, since it is in this form that the main amount of vitamin B 12 enters the human body, without losing sight of the fact that it is not synonymous with B 12, and several other compounds also have B 12 - vitamin activity. Vitamin B 12 is also called Castle's extrinsic factor.

B 12 has the most complex chemical structure compared to other vitamins, the basis of which is the corrin ring. Corrin is in many ways similar to porphyrin (a complex chemical structure that is part of heme, chlorophyll and cytochromes), but differs from porphyrin in that two pyrrole rings in the composition of corrin are directly connected to each other, and not by a methylene bridge. The cobalt ion is located in the center of the corrin structure. Cobalt forms four coordination bonds with nitrogen atoms. Another coordination bond connects cobalt with dimethylbenzimidazole nucleotide. The last, sixth coordination bond of cobalt remains free: it is through this bond that the cyano group, hydroxyl group, methyl or 5 "-deoxyadenosyl residue is added to form four variants of vitamin B 12, respectively. The carbon-covalent covalent bond in the structure of cyanocobalamin is the only one known in living nature is an example of a covalent bond transition metal-carbon.

Conventionally, the chemical reactions of complexes are divided into exchange, redox, isomerization, and coordinated ligands.

The primary dissociation of complexes into the inner and outer spheres determines the course of the reactions of exchange of outer-sphere ions:

Xm + mNaY = Ym + mNaX.

The components of the inner sphere of the complexes can also participate in exchange processes involving both the ligands and the complexing agent. To characterize the substitution reactions of ligands or the central metal ion, the notation and terminology proposed by K. Ingold for reactions of organic compounds (Fig. 42), nucleophilic S N and electrophilic S E substitutions:

Z + Y = z + X S N

Z + M"= z + M S E .

According to the mechanism of the substitution reaction, they are divided (Fig. 43) into associative ( S N 1 and S E 1 ) and dissociative ( S N 2 and S E 2 ), which differ in the transition state with an increased and decreased coordination number.

Assigning the reaction mechanism to associative or dissociative is a difficult experimentally achievable task of identifying an intermediate with a reduced or increased coordination number. In this regard, the reaction mechanism is often judged on the basis of indirect data on the effect of the concentration of reagents on the reaction rate, changes in the geometric structure of the reaction product, etc.

To characterize the rate of ligand substitution reactions in complexes, the 1983 Nobel laureate G. Taube (Fig. 44) suggested using the terms "labile" and "inert" depending on the time of the ligand substitution reaction less or more than 1 minute. The terms labile or inert are characteristics of the kinetics of ligand substitution reactions and should not be confused with thermodynamic characteristics of the stability or instability of complexes.

The lability or inertness of the complexes depends on the nature of the complexing ion and the ligands. According to the ligand field theory:

1. Octahedral complexes 3 d transition metals with a distribution of valence ( n -1) d electrons per sigma*(e g ) of loosening MOs are labile.

4- (t 2g 6 e g 1) + H 2 O= 3- +CN-.

Moreover, the lower the value of the energy of stabilization by the crystal field of the complex, the greater its lability.

2. Octahedral complexes 3 d transition metals with free sigma* leavening e g orbitals and a uniform distribution of valence ( n -1) d electrons in t 2 g orbitals (t 2 g 3, t 2 g 6) are inert.

[ Co III (CN ) 6 ] 3- (t 2 g 6 e g 0 ) + H 2 O =

[ Cr III (CN ) 6 ] 3- (t 2 g 3 e g 0 ) + H 2 O =

3. Plano-square and octahedral 4 d and 5d transition metals that do not have electrons per sigma* loosening MO are inert.

2+ + H 2 O =

2+ + H 2 O =

The influence of the nature of ligands on the rate of ligand substitution reactions is considered within the framework of the “mutual influence of ligands” model. A special case of the model of mutual influence of ligands is formulated in 1926 by I.I. Chernyaev the concept of trans-influence (Fig. 45) - "the lability of the ligand in the complex depends on the nature of the trans-located ligand" - and propose a series of trans-influence ligands: CO , CN - , C 2 H 4 > PR 3 , H - > CH 3 - , SC (NH 2 ) 2 > C 6 H 5 - , NO 2 - , I - , SCN - > Br - , Cl - > py , NH 3 , OH - , H 2 O .

The concept of trans-influence made it possible to substantiate the rules of thumb:

1. Peyronet's rule- under the action of ammonia or amines on tetrachloroplatinate ( II ) potassium is always obtained dichlordiaminplatinum cis-configuration:

2 - + 2NH 3 \u003d cis - + 2Cl -.

Since the reaction proceeds in two stages and the chloride ligand has a large trans effect, the substitution of the second chloride ligand for ammonia occurs with the formation of cis-[ Pt (NH 3) 2 Cl 2]:

2- + NH 3 \u003d -

NH 3 \u003d cis -.

2. Jergensen's rule - under the action of hydrochloric acid on platinum tetrammine chloride ( II ) or similar compounds, dichlorodiammineplatinum trans-configuration is obtained:

[Pt (NH 3 ) 4 ] 2+ + 2 HCl = trans-[Pt (NH 3 ) 2 Cl 2 ] + 2 NH 4 Cl.

In accordance with the series of trans influences of ligands, substitution of the second ammonia molecule for a chloride ligand leads to the formation of trans-[ Pt (NH 3 ) 2 Cl 2].

3. Thiourea Kurnakov reaction - various products of the reaction of thiourea with geometric isomers of trans-[ Pt (NH 3 ) 2 Cl 2 ] and cis-[Pt (NH 3 ) 2 Cl 2 ]:

cis - + 4Thio \u003d 2+ + 2Cl - + 2NH 3.

The different nature of the reaction products is associated with the high trans effect of thiourea. The first stage of the reactions is the replacement of thiourea chloride ligands with the formation of trans- and cis-[ Pt (NH 3 ) 2 (Thio ) 2 ] 2+ :

trans-[ Pt (NH 3 ) 2 Cl 2 ] + 2 Thio = trans-[ Pt (NH 3 ) 2 (Thio ) 2 ] 2+

cis - + 2Thio = cis - 2+.

In cis-[ Pt (NH 3 ) 2 (Thio ) 2 ] 2+ two ammonia molecules trans to thiourea undergo further substitution, which leads to the formation 2+ :

cis - 2+ + 2Thio \u003d 2+ + 2NH 3.

In trans-[ Pt (NH 3 ) 2 (Thio ) 2 ] 2+ two ammonia molecules with a small trans effect are located in the trans position to each other and therefore are not replaced by thiourea.

The patterns of trans-influence were discovered by I.I. Chernyaev when studying ligand substitution reactions in square-planar platinum complexes ( II ). Subsequently, it was shown that the trans effect of ligands also manifests itself in complexes of other metals ( Pt(IV), Pd(II), Co(III), Cr(III), Rh(III), Ir(III )) and other geometric structures. True, the series of the trans-effect of ligands for different metals are somewhat different.

It should be noted that trance influence is kinetic effect- the greater the trans-influence of this ligand, the faster the replacement of another ligand, which is in relation to it in the trans-position.

Along with the kinetic effect of trans-influence, in the middle XX century A.A. Grinberg and Yu.N. Kukushkin established the dependence of the trans effect of the ligand L from the ligand in the cis position to L . Thus, the study of the rate of substitution reaction Cl- ammonia in platinum complexes ( II):

[PtCl 4] 2- + NH 3 = [PtNH 3 Cl 3] - + Cl - K = 0.42 . 10 4 l/mol. With

[PtNH 3 Cl 3] - + NH 3 \u003d cis-[Pt (NH 3) 2 Cl 2] + Cl - K = 1.14 . 10 4 l/mol. With

trans-[ Pt (NH 3 ) 2 Cl 2 ] + NH 3 = [ Pt (NH 3 ) 3 Cl ] + + Cl - K = 2.90 . 10 4 l/mol. With

showed that the presence of one or two ammonia molecules in the cis-position to the chloride ligand being replaced leads to a successive increase in the reaction rate. This kinetic effect is called cis influence. At present, both kinetic effects of the influence of the nature of ligands on the rate of ligand substitution reactions (trans- and cis-effects) are combined in a common concept mutual influence of ligands.

The theoretical substantiation of the effect of the mutual influence of ligands is closely connected with the development of ideas about the chemical bond in complex compounds. In the 30s XX century A.A. Grinberg and B.V. Nekrasov considered the trans-influence within the framework of the polarization model:

1. The trans effect is characteristic of complexes whose central metal ion has a high polarizability.

2. The trans activity of ligands is determined by the mutual polarization energy of the ligand and the metal ion. For a given metal ion, the trans effect of a ligand is determined by its polarizability and distance from the central ion.

The polarization model agrees with experimental data for complexes with simple anionic ligands, for example, halide ions.

In 1943 A.A. Greenberg suggested that the trans activity of ligands is related to their reducing properties. The shift of the electron density from the trans-active ligand to the metal reduces the effective charge of the metal ion, which leads to a weakening of the chemical bond with the trans-located ligand.

The development of ideas about the trans effect is associated with the high trans activity of ligands based on unsaturated organic molecules, like ethylene in [ Pt (C 2 H 4 ) Cl 3 ] - . According to Chatt and Orgel (Fig. 46), this is due topi-the dative interaction of such ligands with the metal and the associative mechanism of substitution reactions for trans-located ligands. Coordination to the metal ion of the attacking ligand Z leads to the formation of a five-coordinate trigonal-bipyramidal intermediate, followed by rapid cleavage of the outgoing ligand X. The formation of such an intermediate is facilitated bypi-dative ligand-metal ligand interaction Y , which reduces the electron density of the metal and reduces the activation energy of the transition state with subsequent rapid substitution of the X ligand.

Along with p acceptor (C 2 H 4, CN -, CO ...) ligands that form a dative ligand-metal chemical bond have a high trans-influence andsdonor ligands: H - , CH 3 - , C 2 H 5 - ... The trans effect of such ligands is determined by the donor-acceptor interaction of the ligand X with the metal, which lowers its electron density and weakens the bond between the metal and the outgoing ligand Y .

Thus, the position of ligands in the trans activity series is determined by the combined action of sigma donor and pi-properties of ligands - sigma- donor and pi-the acceptor properties of the ligand enhance its trans effect, whilepi-donor - weaken. Which of these components of the ligand-metal interaction prevails in the trans effect is judged on the basis of quantum-chemical calculations of the electronic structure of the transition state of the reaction.

The main substitution reaction in aqueous solutions - the exchange of water molecules (22) - was studied for a large number of metal ions (Fig. 34). The exchange of water molecules in the coordination sphere of a metal ion with the bulk of water molecules present as a solvent proceeds very quickly for most metals, and therefore the rate of such a reaction was studied mainly by the relaxation method. The method consists in disturbing the equilibrium of the system, for example, by a sharp increase in temperature. Under new conditions (higher temperature), the system will no longer be in equilibrium. Then measure the rate of equilibrium. If it is possible to change the temperature of the solution within 10 -8 sec, then it is possible to measure the rate of a reaction that requires a time interval greater than 10 -8 sec.

It is also possible to measure the rate of substitution of coordinated water molecules in various metal ions by ligands SO 2-4, S 2 O 3 2- , EDTA, etc. (26). The rate of such a reaction

depends on the concentration of the hydrated metal ion and does not depend on the concentration of the incoming ligand, which makes it possible to use the first-order equation (27) to describe the velocity of these systems. In many cases, the rate of reaction (27) for a given metal ion does not depend on the nature of the incoming ligand (L), be it H 2 O molecules or SO 4 2- , S 2 O 3 2- , or EDTA ions.

This observation, and the fact that the rate equation for this process does not include the concentration of the incoming ligand, suggests that these reactions proceed by a mechanism in which the slow step is to break the bond between the metal ion and water. The resulting compound is then likely to rapidly coordinate nearby ligands.

In sec. 4 of this chapter, it was indicated that more highly charged hydrated metal ions, such as Al 3+ and Sc 3+ , exchange water molecules more slowly than M 2+ and M + ions; this gives grounds to assume that bond breaking plays an important role in the stage that determines the rate of the entire process. The conclusions obtained in these studies are not conclusive, but they give reason to believe that S N 1 processes are important in substitution reactions of hydrated metal ions.

Probably the most studied complex compounds are cobalt(III) ammines. Their stability, ease of preparation, and slow reactions with them make them particularly suitable for kinetic studies. Since the studies of these complexes were carried out exclusively in aqueous solutions, it is first necessary to consider the reactions of these complexes with solvent molecules - water. It was found that, in general, ammonia or amine molecules coordinated to the Co(III) ion are so slowly replaced by water molecules that substitution of ligands other than amines is usually considered.

The rate of reactions of type (28) was studied and found to be of the first order with respect to the cobalt complex (X is one of many possible anions).

Since in aqueous solutions the concentration of H 2 O is always approximately 55.5 M, then it is impossible to determine the effect of changing the concentration of water molecules on the reaction rate. The rate equations (29) and (30) for an aqueous solution are experimentally indistinguishable, since k is simply equal to k" = k". Therefore, it is impossible to tell from the reaction rate equation whether H 2 O will participate in the step that determines the rate of the process. The answer to the question whether this reaction proceeds according to the S N 2 mechanism with the replacement of the X ion by the H 2 O molecule or according to the S N 1 mechanism, which first involves dissociation followed by the addition of the H 2 O molecule, must be obtained using other experimental data.

This problem can be solved by two types of experiments. Hydrolysis rate (substitution of one Cl ion per water molecule) trance- + about 10 3 times the rate of hydrolysis 2+ . An increase in the charge of the complex leads to strengthening of the metal-ligand bonds, and, consequently, to inhibition of the breaking of these bonds. The attraction of incoming ligands and the facilitation of the substitution reaction should also be taken into account. Since a decrease in the rate was found as the charge of the complex increased, in this case a dissociative process (S N 1) seems more likely.

Another way of proof is based on the study of the hydrolysis of a series of complexes similar to trance- + . In these complexes, the ethylenediamine molecule is replaced by similar diamines, in which the hydrogen atoms at the carbon atom are replaced by CH 3 groups. Complexes containing substituted diamines react faster than the ethylenediamine complex. Replacing hydrogen atoms with CH 3 groups increases the volume of the ligand, which makes it difficult for another ligand to attack the metal atom. These steric hindrances slow down the reaction by the S N 2 mechanism. The presence of bulky ligands near the metal atom promotes the dissociative process, since the removal of one of the ligands reduces their accumulation at the metal atom. The observed increase in the rate of hydrolysis of complexes with bulky ligands is good evidence that the reaction proceeds according to the S N 1 mechanism.

So, as a result of numerous studies of Co(II) acidoamine complexes, it turned out that the replacement of acid groups by water molecules is a dissociative process in nature. The cobalt atom-ligand bond lengthens to a certain critical value before water molecules begin to enter the complex. In complexes with a charge of 2+ and higher, the breaking of the cobalt-ligand bond is very difficult, and the entry of water molecules begins to play a more important role.

It was found that the replacement of the acido group (X -) in the cobalt(III) complex with a group other than the H 2 O molecule (31) first proceeds through its substitution by the molecule

solvent - water, followed by its replacement with a new group Y (32).

Thus, in many reactions with cobalt(III) complexes, the rate of reaction (31) is equal to the rate of hydrolysis (28). Only the hydroxyl ion differs from other reagents in terms of reactivity with Co(III) amines. It reacts very quickly with amminic complexes of cobalt(III) (about 10 6 times faster than water) according to the type of reaction basic hydrolysis (33).

This reaction was found to be first order with respect to the substituting ligand OH - (34). The overall second order of the reaction and the unusually fast progression of the reaction suggest that the OH ion is an exceptionally effective nucleophilic reagent with respect to Co(III) complexes and that the reaction proceeds via the S N 2 mechanism through the formation of an intermediate.

However, this property of OH - can also be explained by another mechanism [equations (35), (36)]. In reaction (35), complex 2+ behaves like an acid (according to Bronsted), giving complex + , which is amido-(containing)-compound - a base corresponding to an acid 2+.

Then the reaction proceeds according to the mechanism S N 1 (36) with the formation of a five-coordination intermediate compound, which then reacts with solvent molecules, which leads to the final reaction product (37). This reaction mechanism is consistent with the second-order reaction rate and corresponds to the S N 1 mechanism. Since the reaction in the rate-determining step involves a base conjugated to the initial acid complex, this mechanism is given the designation S N 1CB.

It is very difficult to determine which of these mechanisms best explains experimental observations. However, there is strong evidence supporting the S N 1CB hypothesis. The best arguments in favor of this mechanism are as follows: octahedral Co(III) complexes generally react according to the dissociative S N 1 mechanism, and there are no convincing arguments why the OH ion should cause the S N 2 process. It has been established that the hydroxyl ion is a weak nucleophilic reagent in reactions with Pt(II), and therefore its unusual reactivity with Co(III) seems unreasonable. Reactions with cobalt(III) compounds in non-aqueous media provide excellent evidence for the formation of five-coordination intermediates provided for by the S N 1 CB mechanism.

The final proof is the fact that in the absence of N - H bonds in the Co(III) complex, it slowly reacts with OH - ions. This, of course, gives grounds to believe that the acid-base properties of the complex are more important for the reaction rate than the nucleophilic properties of OH. This reaction of the basic hydrolysis of ammine complexes of Co (III) is an illustration of the fact that kinetic data can often be interpreted in more than one way, and In order to exclude this or that possible mechanism, it is necessary to carry out a rather subtle experiment.

At present, substitution reactions of a large number of octahedral compounds have been studied. If we consider their reaction mechanisms, then the dissociative process is most often encountered. This result is not unexpected since the six ligands leave little space around the central atom for other groups to attach to it. Only a few examples are known where the occurrence of a seven-coordination intermediate has been proven or the effect of an incorporating ligand has been detected. Therefore, the S N 2 mechanism cannot be completely rejected as a possible pathway for substitution reactions in octahedral complexes.

Chapter 17

17.1. Basic definitions

In this chapter, you will be introduced to a special group of complex substances called comprehensive(or coordinating) connections.

Currently, a strict definition of the concept " complex particle" No. The following definition is usually used.

For example, a hydrated copper ion 2 is a complex particle, since it actually exists in solutions and some crystalline hydrates, it is formed from Cu 2 ions and H 2 O molecules, water molecules are real molecules, and Cu 2 ions exist in crystals of many copper compounds. On the contrary, the SO 4 2 ion is not a complex particle, since although O 2 ions occur in crystals, the S 6 ion does not exist in chemical systems.

Examples of other complex particles: 2 , 3 , , 2 .

At the same time, NH 4 and H 3 O ions are classified as complex particles, although H ions do not exist in chemical systems.

Sometimes complex particles are called complex chemical particles, all or part of the bonds in which are formed according to the donor-acceptor mechanism. This is true in most complex particles, but, for example, in potassium alum SO 4 in complex particle 3, the bond between Al and O atoms is indeed formed according to the donor-acceptor mechanism, while in the complex particle there is only electrostatic (ion-dipole) interaction. This is confirmed by the existence in iron ammonium alum of a complex particle similar in structure, in which only ion-dipole interaction is possible between water molecules and the NH 4 ion.

By charge, complex particles can be cations, anions, and also neutral molecules. Complex compounds containing such particles can belong to different classes of chemicals (acids, bases, salts). Examples: (H 3 O) - acid, OH - base, NH 4 Cl and K 3 - salts.

Typically, the complexing agent is an atom of an element that forms a metal, but it can also be an atom of oxygen, nitrogen, sulfur, iodine, and other elements that form non-metals. The oxidation state of the complexing agent may be positive, negative, or zero; when a complex compound is formed from simpler substances, it does not change.

Ligands can be particles that, before the formation of a complex compound, were molecules (H 2 O, CO, NH 3, etc.), anions (OH, Cl, PO 4 3, etc.), as well as a hydrogen cation. Distinguish unidentate or monodentate ligands (linked to the central atom through one of its atoms, that is, by one -bond), bidentate(connected to the central atom through two of their atoms, that is, by two -bonds), tridentate etc.

If the ligands are unidentate, then the coordination number is equal to the number of such ligands.

The cn depends on the electronic structure of the central atom, its degree of oxidation, the size of the central atom and ligands, the conditions for the formation of the complex compound, temperature, and other factors. CN can take values ​​from 2 to 12. Most often it is equal to six, somewhat less often - four.

There are also complex particles with several central atoms.

Two types of structural formulas of complex particles are used: indicating the formal charge of the central atom and ligands, or indicating the formal charge of the entire complex particle. Examples:

To characterize the shape of a complex particle, the idea of ​​a coordination polyhedron (polyhedron) is used.

Coordination polyhedra also include a square (KN = 4), a triangle (KN = 3), and a dumbbell (KN = 2), although these figures are not polyhedra. Examples of coordination polyhedra and correspondingly shaped complex particles for the most common CN values ​​are shown in Figs. 1.

17.2. Classification of complex compounds

How chemicals complex compounds are divided into ionic (they are sometimes called ionogenic) and molecular ( non-ionic) connections. Ionic complex compounds contain charged complex particles - ions - and are acids, bases or salts (see § 1). Molecular complex compounds consist of uncharged complex particles (molecules), for example: or - it is difficult to assign them to any main class of chemicals.

The complex particles that make up complex compounds are quite diverse. Therefore, several classification features are used for their classification: the number of central atoms, the type of ligand, the coordination number, and others.

According to the number of central atoms complex particles are divided into single-core And multi-core. The central atoms of multinuclear complex particles can be linked to each other either directly or through ligands. In both cases, the central atoms with ligands form a single inner sphere of the complex compound:

According to the type of ligands, complex particles are divided into

1) Aquacomplexes, that is, complex particles in which water molecules are present as ligands. Cationic aquacomplexes m are more or less stable, anionic aquacomplexes are unstable. All crystalline hydrates are compounds containing aqua complexes, for example:

Mg(ClO 4) 2. 6H 2 O is actually (ClO 4) 2 ;
BeSO4. 4H 2 O is actually SO 4 ;
Zn(BrO 3) 2 . 6H 2 O is actually (BrO 3) 2 ;
CuSO4. 5H 2 O is actually SO 4 . H2O.

2) Hydroxocomplexes, that is, complex particles in which hydroxyl groups are present as ligands, which were hydroxide ions before entering the complex particle, for example: 2 , 3 , .

Hydroxo complexes are formed from aqua complexes that exhibit the properties of cationic acids:

2 + 4OH = 2 + 4H 2 O

3) Ammonia, that is, complex particles in which NH 3 groups are present as ligands (before the formation of a complex particle - ammonia molecules), for example: 2 , , 3 .

Ammonia can also be obtained from aqua complexes, for example:

2 + 4NH 3 \u003d 2 + 4 H 2 O

The color of the solution in this case changes from blue to ultramarine.

4) acidocomplexes, that is, complex particles in which acidic residues of both oxygen-free and oxygen-containing acids are present as ligands (before the formation of a complex particle - anions, for example: Cl, Br, I, CN, S 2, NO 2, S 2 O 3 2 , CO 3 2 , C 2 O 4 2 etc.).

Examples of the formation of acid complexes:

Hg 2 + 4I = 2
AgBr + 2S 2 O 3 2 = 3 + Br

The latter reaction is used in photography to remove unreacted silver bromide from photographic materials.
(When developing photographic film and photographic paper, the unexposed part of the silver bromide contained in the photographic emulsion is not restored by the developer. To remove it, this reaction is used (the process is called "fixing", since the unremoved silver bromide gradually decomposes in the light, destroying the image)

5) Complexes in which hydrogen atoms are ligands are divided into two completely different groups: hydride complexes and complexes included in the composition onium connections.

In the formation of hydride complexes - , , - the central atom is an electron acceptor, and the hydride ion is a donor. The oxidation state of hydrogen atoms in these complexes is –1.

In onium complexes, the central atom is an electron donor, and the acceptor is a hydrogen atom in the +1 oxidation state. Examples: H 3 O or - oxonium ion, NH 4 or - ammonium ion. In addition, there are substituted derivatives of such ions: - tetramethylammonium ion, - tetraphenylarsonium ion, - diethyloxonium ion, etc.

6) Carbonyl complexes - complexes in which CO groups are present as ligands (before complex formation - carbon monoxide molecules), for example:,, etc.

7) Anion halide complexes are complexes of type .

Other classes of complex particles are also distinguished according to the type of ligands. In addition, there are complex particles with ligands of various types; the simplest example is aqua hydroxocomplex.

17.3. Fundamentals of the nomenclature of complex compounds

The formula of a complex compound is compiled in the same way as the formula of any ionic substance: the formula of the cation is written in the first place, and the anion in the second.

The formula of a complex particle is written in square brackets in the following sequence: the symbol of the complexing element is placed first, then the formulas of the ligands that were cations before the formation of the complex, then the formulas of the ligands that were neutral molecules before the formation of the complex, and after them the formulas of the ligands, former before the formation of the complex by anions.

The name of a complex compound is built in the same way as the name of any salt or base (complex acids are called hydrogen or oxonium salts). The name of the compound includes the name of the cation and the name of the anion.

The name of the complex particle includes the name of the complexing agent and the names of the ligands (the name is written in accordance with the formula, but from right to left. For complexing agents in cations, Russian element names are used, and in anions, Latin ones.

Names of the most common ligands:

H 2 O - aqua	Cl - chloro	SO 4 2 - sulfate	OH - hydroxo
CO - carbonyl	Br - bromo	CO 3 2 - carbonate	H - hydrido
NH 3 - ammine	NO 2 - nitro	CN - cyano	NO - nitroso
NO - nitrosyl	O 2 - oxo	NCS - thiocyanato	H + I - hydro

Examples of names of complex cations:

Examples of names of complex anions:

2 - tetrahydroxozincate ion
3 – di(thiosulfato)argentate(I)-ion
3 – hexacyanochromate(III)-ion
– tetrahydroxodiquaaluminate ion
– tetranitrodiamminecobaltate(III)-ion
3 – pentacyanoaquaferrate(II)-ion

Examples of the names of neutral complex particles:

More detailed nomenclature rules are given in reference books and special manuals.

17.4. Chemical bond in complex compounds and their structure

In crystalline complex compounds with charged complexes, the bond between the complex and the outer sphere ions is ionic, while the bonds between the remaining particles of the outer sphere are intermolecular (including hydrogen bonds). In molecular complex compounds, the bond between the complexes is intermolecular.

In most complex particles, the bonds between the central atom and the ligands are covalent. All or part of them are formed according to the donor-acceptor mechanism (as a result, with a change in formal charges). In the least stable complexes (for example, in the aqua complexes of alkali and alkaline earth elements, as well as ammonium), ligands are held by electrostatic attraction. The bond in complex particles is often referred to as a donor-acceptor or coordination bond.

Let us consider its formation using the iron(II) aquacation as an example. This ion is formed by the reaction:

FeCl 2cr + 6H 2 O = 2 + 2Cl

The electronic formula of the iron atom is 1 s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6. Let's make a diagram of the valence sublevels of this atom:

When a doubly charged ion is formed, the iron atom loses two 4 s-electron:

The iron ion accepts six electron pairs of oxygen atoms of six water molecules into free valence orbitals:

A complex cation is formed, the chemical structure of which can be expressed by one of the following formulas:

The spatial structure of this particle is expressed by one of the spatial formulas:

The shape of the coordination polyhedron is an octahedron. All Fe-O bonds are the same. Supposed sp 3 d 2 - hybridization of iron atom AO. The magnetic properties of the complex indicate the presence of unpaired electrons.

If FeCl 2 is dissolved in a solution containing cyanide ions, then the reaction proceeds

FeCl 2cr + 6CN = 4 + 2Cl.

The same complex is also obtained by adding a solution of potassium cyanide KCN to a FeCl 2 solution:

2 + 6CN \u003d 4 + 6H 2 O.

This suggests that the cyanide complex is stronger than the aquacomplex. In addition, the magnetic properties of the cyanide complex indicate the absence of unpaired electrons from the iron atom. All this is due to a slightly different electronic structure of this complex:

The "stronger" CN ligands form stronger bonds with the iron atom, the energy gain is enough to "break" the Hund's rule and release 3 d-orbitals for lone pairs of ligands. The spatial structure of the cyanide complex is the same as that of the aquacomplex, but the type of hybridization is different - d 2 sp 3 .

The "strength" of the ligand depends primarily on the electron density of the cloud of the lone pair of electrons, that is, it increases with decreasing atom size, with decreasing principal quantum number, depends on the type of EO hybridization, and on some other factors. The most important ligands can be lined up in order of increasing their "strength" (a kind of "activity series" of ligands), this series is called spectrochemical series of ligands:

I; Br; : SCN, Cl, F, OH, H 2 O; : NCS, NH3; SO 3 S : 2 ; : CN, CO

For complexes 3 and 3, the formation schemes look as follows:

For complexes with CN = 4, two structures are possible: a tetrahedron (in the case sp 3-hybridization), for example, 2 , and a flat square (in the case of dsp 2 hybridization), for example, 2 .

17.5. Chemical properties of complex compounds

For complex compounds, first of all, the same properties are characteristic as for ordinary compounds of the same classes (salts, acids, bases).

If the compound is an acid, then it is a strong acid; if it is a base, then the base is strong. These properties of complex compounds are determined only by the presence of H 3 O or OH ions. In addition, complex acids, bases and salts enter into the usual exchange reactions, for example:

SO 4 + BaCl 2 \u003d BaSO 4 + Cl 2
FeCl 3 + K 4 = Fe 4 3 + 3KCl

The last of these reactions is used as a qualitative reaction for Fe 3 ions. The resulting ultramarine insoluble substance is called "prussian blue" [the systematic name is iron(III)-potassium hexacyanoferrate(II)].

In addition, the complex particle itself can enter into the reaction, and the more actively, the less stable it is. Usually these are ligand substitution reactions occurring in solution, for example:

2 + 4NH 3 \u003d 2 + 4H 2 O,

as well as acid-base reactions such as

2 + 2H 3 O = + 2H 2 O
2 + 2OH = + 2H 2 O

Formed in these reactions, after isolation and drying, it turns into zinc hydroxide:

Zn(OH) 2 + 2H 2 O

The last reaction is the simplest example of the decomposition of a complex compound. In this case, it runs at room temperature. Other complex compounds decompose when heated, for example:

SO4. H 2 O \u003d CuSO 4 + 4NH 3 + H 2 O (above 300 o C)
4K 3 \u003d 12KNO 2 + 4CoO + 4NO + 8NO 2 (above 200 o C)
K 2 \u003d K 2 ZnO 2 + 2H 2 O (above 100 o C)

To assess the possibility of a ligand substitution reaction, a spectrochemical series can be used, guided by the fact that stronger ligands displace weaker ones from the inner sphere.

17.6. Isomerism of complex compounds

Isomerism of complex compounds is related
1) with possible different arrangement of ligands and outer-sphere particles,
2) with a different structure of the most complex particle.

The first group includes hydrated(in general solvate) And ionization isomerism, to the second - spatial And optical.

Hydrate isomerism is associated with the possibility of different distribution of water molecules in the outer and inner spheres of the complex compound, for example: (red-brown color) and Br 2 (blue color).

Ionization isomerism is associated with the possibility of different distribution of ions in the outer and inner spheres, for example: SO 4 (purple) and Br (red). The first of these compounds forms a precipitate, reacting with a solution of barium chloride, and the second - with a solution of silver nitrate.

Spatial (geometric) isomerism, otherwise called cis-trans isomerism, is characteristic of square and octahedral complexes (it is impossible for tetrahedral ones). Example: cis-trans square complex isomerism

Optical (mirror) isomerism essentially does not differ from optical isomerism in organic chemistry and is characteristic of tetrahedral and octahedral complexes (impossible for square ones).