R&D: Elementary stages involving coordination and organometallic compounds in solutions and on the surface of metals and oxides. Reactivity of complexes
Reactions of substitution, addition or elimination of ligands, as a result of which the coordination sphere of the metal changes.
In a broad sense, substitution reactions are understood as the processes of substitution of some ligands in the coordination sphere of the metal by others.
Dissociative (D) mechanism. The two-stage process in the limiting case proceeds through an intermediate with a smaller coordination number:
ML6<->+L; + Y --» ML5Y
Associative (A) mechanism. The two-stage process is characterized by the formation of an intermediate with a large coordination number: ML6 + Y = ; = ML5Y + L
Reciprocal exchange mechanism (I). Most of the exchange reactions proceed according to this mechanism. The process is single-stage and is not accompanied by the formation of an intermediate. In the transition state, the reagent and the leaving group are bound to the reaction center, enter its nearest coordination sphere, and during the reaction one group is displaced by another, the exchange of two ligands:
ML6 + Y = = ML5Y+L
internal mechanism. This mechanism characterizes the process of ligand substitution at the molecular level.
2. Features of the properties of lanthanides (Ln) associated with the effect of lanthanide compression. Ln 3+ compounds: oxides, hydroxides, salts. Other oxidation states. Examples of reducing properties of Sm 2+ , Eu 2+ and oxidizing properties of Ce 4+ , Pr 4+ .
The monotonic decrease in atomic and ionic radii as one moves along the 4f-element series is called lanthanide contraction. I. It leads to the fact that the atomic radii of the 5d-transition elements of the fourth (hafnium) and fifth (tantalum) groups following the lanthanides turn out to be practically equal to the radii of their electronic counterparts from the fifth period: zirconium and niobium, respectively, and the chemistry of heavy 4d- and 5d-metals has a lot in common. Another consequence of f-compression is the closeness of the ionic radius of yttrium to the radii of the heavy f-elements: dysprosium, holmium, and erbium.
All rare earth elements form stable oxides in the +3 oxidation state. They are refractory crystalline powders that slowly absorb carbon dioxide and water vapor. Oxides of most elements are obtained by calcining hydroxides, carbonates, nitrates, oxalates in air at a temperature of 800-1000 °C.
Form oxides M2O3 and hydroxides M(OH)3
Only scandium hydroxide is amphoteric
Oxides and hydroxides readily dissolve in acids
Sc2O3 + 6HNO3 = 2Sc(NO3)3 + 3H2O
Y(OH)3 + 3HCl = YCl3 + 3H2O
Only scandium compounds hydrolyze in aqueous solution.
Cl3 ⇔ Cl2 + HCl
All halides are known in the +3 oxidation state. All are hardboilers.
Fluorides are poorly soluble in water. Y(NO3)3 + 3NaF = YF3↓+ 3NaNO3
Introduction to work
The relevance of the work. Complexes of porphyrins with metals in high oxidation states can coordinate bases much more efficiently than M2+ complexes and form mixed coordination compounds in which, in the first coordination sphere of the central metal atom, along with the macrocyclic ligand, there are noncyclic acidoligands, and sometimes coordinated molecules. The issues of compatibility of ligands in such complexes are extremely important, since it is in the form of mixed complexes that porphyrins perform their biological functions. In addition, reactions of reversible addition (transfer) of base molecules, characterized by moderately high equilibrium constants, can be successfully used for the separation of mixtures of organic isomers, for quantitative analysis, for the purposes of ecology and medicine. Therefore, studies of the quantitative characteristics and stoichiometry of additional coordination equilibria on metalloporphyrins (MPs) and substitution of simple ligands in them are useful not only from the point of view of theoretical knowledge of the properties of metalloporphyrins as complex compounds, but also for solving the practical problem of searching for receptors and carriers of small molecules or ions. So far, there are practically no systematic studies on complexes of highly charged metal ions.
Objective. This work is devoted to the study of the reactions of mixed porphyrin-containing complexes of highly charged metal cations Zr IV , Hf IV , Mo V and W V with bioactive N-bases: imidazole (Im), pyridine (Py), pyrazine (Pyz), benzimidazole (BzIm), characterization stability and optical properties of molecular complexes, substantiation of stepwise reaction mechanisms.
Scientific novelty. Methods of modified spectrophotometric titration, chemical kinetics, electronic and vibrational absorption and 1 H NMR spectroscopy were used for the first time to obtain thermodynamic characteristics and substantiate the stoichiometric mechanisms of reactions of N-bases with metal porphyrins with a mixed coordination sphere (X) -, O 2-, TPP - tetraphenylporphyrin dianion). It has been established that in the vast majority of cases, the processes of formation of metalloporphyrin-base supramolecules proceed stepwise and include several reversible and slow irreversible elementary reactions of coordination of base molecules and substitution of acidoligands. For each stage of the stepwise reactions, the stoichiometry, equilibrium or rate constants, base orders of slow reactions were determined, and the products were spectrally characterized (UV, visible spectra for intermediate products and UV, visible and IR for final products). Correlation equations have been obtained for the first time, which make it possible to predict the stability of supramolecular complexes with other bases. The equations are used in this work to discuss the detailed mechanism of substitution of OH - in Mo and W complexes by a base molecule. The properties of MR are described, which determine the prospect of using them for the detection, separation, and quantitative analysis of biologically active bases, such as moderately high stability of supramolecular complexes, clear and fast optical response, low sensitivity threshold, and one-second circulation time.
The practical significance of the work. Quantitative results and substantiation of the stoichiometric mechanisms of molecular complex formation reactions are essential for the coordination chemistry of macroheterocyclic ligands. The dissertation work shows that mixed porphyrin-containing complexes exhibit high sensitivity and selectivity with respect to bioactive organic bases, within a few seconds or minutes they give an optical response suitable for the practical detection of reactions with bases - VOCs, components of drugs and food, due to which are recommended for use as components of base sensors in ecology, food industry, medicine and agriculture.
Approbation of work. The results of the work were reported and discussed at:
IX International Conference on Problems of Solvation and Complex Formation in Solutions, Ples, 2004; XII Symposium on Intermolecular Interactions and Conformations of Molecules, Pushchino, 2004; XXV, XXVI and XXIX Scientific Sessions of the Russian Seminar on the Chemistry of Porphyrins and Their Analogues, Ivanovo, 2004 and 2006; VI School-Conference of young scientists of the CIS countries on the chemistry of porphyrins and related compounds, St. Petersburg, 2005; VIII scientific school - conferences on organic chemistry, Kazan, 2005; All-Russian scientific conference "Natural macrocyclic compounds and their synthetic analogues", Syktyvkar, 2007; XVI International Conference on Chemical Thermodynamics in Russia, Suzdal, 2007; XXIII International Chugaev Conference on Coordination Chemistry, Odessa, 2007; International Conference on Porphyrins and Phtalocyanines ISPP-5, 2008; 38th International Conference on Coordination Chemistry, Israel, 2008.
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. one.
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 alkaline 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 scheme of 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 a decrease in the size of the atom, with a decrease in the 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).
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). The efficiency and strength of the donor-acceptor interaction between a ligand and a complexing agent is determined by their polarizability, i.e., the ability of a particle to transform its electron shells under external influence.
Instability constant:
Knest= 2 /
K mouth \u003d 1 / Knest
Ligand substitution reactions
One of the most important steps in metal complex catalysis, the interaction of the Y substrate with the complex, proceeds via three mechanisms:
a) Replacement of the ligand with a solvent. Usually such a stage is depicted as the dissociation of the complex
The essence of the process in most cases is the replacement of the ligand L by the solvent S, which is then easily replaced by the substrate molecule Y
b) Attachment of a new ligand along a free coordinate with the formation of an associate, followed by dissociation of the substituted ligand
c) Synchronous substitution (type S N 2) without the formation of an intermediate
Ideas about the structure of metalloenzymes and other biocomplex compounds (hemoglobin, cytochromes, cobalamins). Physical and chemical principles of oxygen transport by hemoglobin.
Structural features of metalloenzymes.
Biocomplex compounds vary considerably in stability. The role of the metal in such complexes is highly specific: replacing it even with an element with similar properties leads to a significant or complete loss of physiological activity.
1. B12: contains 4 pyrrole rings, cobalt ion and CN- groups. Promotes the transfer of the H atom to the C atom in exchange for any group, participates in the formation of deoxyribose from ribose.
2. hemoglobin: has a quaternary structure. Four polypeptide chains connected together form an almost regular ball shape, where each chain contacts two chains.
Hemoglobin is a respiratory pigment that gives blood its red color. Hemoglobin is composed of protein and iron porphyrin and carries oxygen from the respiratory organs to body tissues and carbon dioxide from them to the respiratory organs.
Cytochromes- complex proteins (hemoproteins) that carry out stepwise transfer of electrons and / or hydrogen from oxidizable organic substances to molecular oxygen in living cells. This produces an energy-rich ATP compound.
Cobalamins- natural biologically active organocobalt compounds. The structural basis of cobalt is a corrin ring, consisting of 4 pyrrole nuclei, in which the nitrogen atoms are bonded to the central cobalt atom.
Physico-chemical principles of oxygen transport by hemoglobin- Atom (Fe (II)) (one of the components of hemoglobin) is able to form 6 coordination bonds. Of these, four are used to fix the Fe(II) atom itself in the heme, the fifth bond is used to bind the heme to the protein subunit, and the sixth bond is used to bind the O 2 or CO 2 molecule.
Metal-ligand homeostasis and causes of its violation. Mechanism of toxic action of heavy metals and arsenic based on the theory of hard and soft acids and bases (HMBA). Thermodynamic principles of chelation therapy. Mechanism of cytotoxic action of platinum compounds.
In the body, the formation and destruction of biocomplexes from metal cations and bioligands (porphins, amino acids, proteins, polynucleotides), which include donor atoms of oxygen, nitrogen, and sulfur, continuously occur. The exchange with the environment maintains the concentrations of these substances at a constant level, providing metal ligand homeostasis. Violation of the existing balance leads to a number of pathological phenomena - metal surplus and metal deficiency states. An incomplete list of diseases associated with changes in the metal-ligand balance for only one ion, the copper cation, can be cited as an example. Deficiency of this element in the body causes Menkes syndrome, Morfan syndrome, Wilson-Konovalov disease, cirrhosis of the liver, emphysema, aorto- and arteriopathy, anemia. Excessive intake of the cation can lead to a series of diseases of various organs: rheumatism, bronchial asthma, inflammation of the kidneys and liver, myocardial infarction, etc., called hypercupremia. Professional hypercupreosis is also known - copper fever.
The circulation of heavy metals occurs partially in the form of ions or complexes with amino acids, fatty acids. However, the leading role in the transport of heavy metals belongs to proteins that form a strong bond with them.
They are fixed on cell membranes, block the thiol groups of membrane proteins- 50% of them are protein-enzymes that disrupt the stability of the protein-lipid complexes of the cell membrane and its permeability, causing the release of potassium from the cell and the penetration of sodium and water into it.
A similar effect of these poisons, which are actively fixed on red blood cells, leads to disruption of the integrity of erythrocyte membranes, inhibition of aerobic glycolysis and metabolism processes in them in general, and accumulation of hemolytically active hydrogen peroxide due to inhibition of peroxidase in particular, which leads to the development of one of the characteristic symptoms of poisoning by compounds this group - to hemolysis.
The distribution and deposition of heavy metals and arsenic occur in almost all organs. Of particular interest is the ability of these substances to accumulate in the kidneys, which is explained by the rich content of thiol groups in the renal tissue, the presence of a protein in it - metallobionin, which contains a large number of thiol groups, which contributes to the long-term deposition of poisons. The liver tissue, also rich in thiol groups and containing metallobionin, is also distinguished by a high degree of accumulation of toxic compounds of this group. The term of deposit, for example, of mercury can reach 2 months or more.
The excretion of heavy metals and arsenic occurs in different proportions through the kidneys, liver (with bile), mucous membrane of the stomach and intestines (with feces), sweat and salivary glands, lungs, which is usually accompanied by damage to the excretory apparatus of these organs and manifests itself in the corresponding clinical symptoms.
The lethal dose for soluble mercury compounds is 0.5 g, for calomel 1–2 g, for copper sulfate 10 g, for lead acetate 50 g, for white lead 20 g, for arsenic 0.1–0.2 g.
The concentration of mercury in the blood is more than 10 µg/l (1γ%), in the urine more than 100 µg/l (10γ%), the concentration of copper in the blood is more than 1600 µg/l (160γ%), arsenic is more than 250 µg/l (25γ%) %) in the urine.
Chelation therapy is the removal of toxic particles
from the body, based on their chelation
s-element complexonates.
Drugs used to remove
incorporated in the body of toxic
particles are called detoxifiers.
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 exchange reactions 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.
