Structure, classification, nomenclature of complex compounds. complex compounds. Definition, classification Complex compounds briefly

In order to give a more or less accurate definition of what complex compounds are, modern chemistry has to rely on the main provisions of the coordination theory, which was proposed by A. Werner back in 1893. The complexity of this issue lies in the diversity and multiplicity of the most diverse chemical compounds falling under under the definition of complex.

In general terms, complex compounds are those that contain a number of complex particles. Until now, science does not have a strict definition of the concept of "complex particle". The following definition is often used: a complex particle is understood as a complex particle that is capable of independently existing both in a crystal and in solution. It consists of other simple particles, which in turn have the ability to exist independently. Also often under the definition of complex particles fall complex chemical particles in which all bonds or part of them are formed according to the donor-acceptor principle.

A common feature that all complex compounds have is the presence in their structure of a central atom, which has received the name "complexing agent". Given the diversity that these compounds possess, it is not necessary to talk about any common features of this element. Often, the complexing agent is an atom forming a metal. But this is not a strict sign: complex compounds are known in which the central atom is an atom of oxygen, sulfur, nitrogen, iodine and other elements that are bright non-metals. Speaking about the charge of the complexing agent, we can say that it is mostly positive, and in the scientific literature it was called a metal center, but examples are known when the central atom had a negative charge, and even zero.

Accordingly, isolated groups of atoms or individual atoms that are located around the complexing agent are called ligands. These can also be particles that, before entering the composition of the complex compound, were molecules, for example, water (H2O), (CO), nitrogen (NH3) and many others, it can also be anions OH–, PO43–, Cl– , or the hydrogen cation H+.

An attempt to classify complex compounds according to the type of charge of the complex separates these chemical compounds into cationic complexes, which are formed around a positively charged ion of neutral molecules. There are also anionic complexes in which the complexing agent is an atom with a positive. Simple and complex anions are ligands. Neutral complexes can be distinguished as a separate group. Their formation occurs by coordination around the neutral atom of the molecules. Also, this category of complex substances includes compounds formed by simultaneous coordination around a positively charged ion and molecules, and negatively charged ions.

If we take into account the number of places occupied by ligands in the so-called coordination sphere, then monodentate, bidentate and polydentate ligands are determined.

The preparation of complex compounds by various methods allows classification according to the nature of the ligand. Among them, ammoniates are distinguished, in which the ligands are represented by ammonia molecules, aqua complexes, where the ligands are water, carbonyls - carbon monoxide plays the role of a ligand. In addition, there are acid complexes in which the central atom is surrounded by acid residues. If it is surrounded by hydroxide ions, then the compounds are classified as hydroxo complexes.

Complex compounds play an important role in nature. Without them, the life of living organisms is impossible. Also, the use of complex compounds in human activity makes it possible to carry out complex technological operations.

Analytical chemistry, extraction of metals from ores, electroforming, production of varnishes and paints - this is only a short list of industries in which complex chemicals have been used.

Formed from other, simpler particles, also capable of independent existence. Sometimes complex particles are called complex chemical particles, all or part of the bonds in which are formed along.

complexing agent is the central atom of a complex particle. Typically, the complexing agent is an atom of a metal-forming element, but it can also be an atom of oxygen, nitrogen, sulfur, iodine, and other non-metal-forming elements. The complexing agent is usually positively charged and in this case is referred to in modern scientific literature metal center; the charge of the complexing agent can also be negative or equal to zero.

Ligand denticity is determined by the number of coordination sites occupied by the ligand in the coordination sphere of the complexing agent. There are monodentate (unidentate) ligands connected to the central atom through one of its atoms, that is, one covalent bond), bidentate (connected to the central atom through two of its atoms, that is, two bonds), tri-, tetradentate, etc. .

Coordination polyhedron- an imaginary molecular polyhedron, in the center of which there is a complexing atom, and in the vertices - particles of ligands directly associated with the central atom.

Tetracarbonylnickel
- dichlorodiammineplatinum(II)

According to the number of places occupied by ligands in the coordination sphere

1) Monodentate ligands. Such ligands are neutral (molecules H 2 O, NH 3, CO, NO, etc.) and charged (ions CN - , F - , Cl - , OH - , SCN - , S 2 O 3 2 - and others).

2) Bidentate ligands. Examples are ligands: aminoacetic acid ion H 2 N - CH 2 - COO - , oxalate ion - O - CO - CO - O - , carbonate ion CO 3 2 - , sulfate ion SO 4 2 - .

3) Polydentate ligands. For example, complexones are organic ligands containing in their composition several groups -C≡N or -COOH (ethylenediaminetetraacetic acid - EDTA). Cyclic complexes formed by some polydentate ligands are referred to as chelate complexes (hemoglobin, etc.).

By the nature of the ligand

1) Ammonia- complexes in which ammonia molecules serve as ligands, for example: SO 4, Cl 3, Cl 4, etc.

2) Aquacomplexes- in which water acts as a ligand: Cl 2, Cl 3, etc.

3) carbonyls- complex compounds in which the ligands are molecules of carbon monoxide (II): , .

4) acidocomplexes- complexes in which ligands are acid residues. These include complex salts: K 2 , complex acids: H 2 , H 2 .

5) Hydroxocomplexes- complex compounds in which hydroxide ions act as ligands: Na 2, Na 2, etc.

Nomenclature

1) In the name of the complex compound, the negatively charged part is first indicated - anion, then the positive part - cation.

2) The name of the complex part begins with an indication of the composition of the inner sphere. In the inner sphere, first of all, ligands are called anions, adding the ending "o" to their Latin name. For example: Cl - - chloro, CN - - cyano, SCN - - thiocyanato, NO 3 - - nitrate, SO 3 2 - - sulfito, OH - - hydroxo, etc. In this case, the terms are used: for coordinated ammonia - ammine, for water - aqua, for carbon monoxide (II) - carbonyl.

(NH 4) 2 - ammonium dihydroxotetrachloroplatinate (IV)

[Cr(H 2 O) 3 F 3] - trifluorotriaquachrome

[Сo (NH 3) 3 Cl (NO 2) 2] - dinitritechlorotriamminecobalt

Cl 2 - dichlorotetraammineplatinum(IV) chloride

NO 3 - tetraaqualitium nitrate

Story

The founder of the coordination theory of complex compounds is the Swiss chemist Alfred Werner (1866-1919). Werner's 1893 coordination theory was the first attempt to explain the structure of complex compounds. This theory was proposed before the discovery of the electron by Thomson in 1896, and before the development of the electronic theory of valence. Werner did not have any instrumental research methods at his disposal, and all his research was done by interpreting simple chemical reactions.

The ideas about the possibility of the existence of "additional valences", which arose in the study of quaternary amines, Werner also applies to "complex compounds". In his 1891 article "On the Theory of Affinity and Valence", Werner defines affinity as "a force emanating from the center of the atom and spreading uniformly in all directions, the geometric expression of which is thus not a certain number of principal directions, but spherical surface. Two years later, in the article "On the Structure of Inorganic Compounds," Werner put forward a coordination theory, according to which complex-forming atoms form the central nucleus in inorganic molecular compounds. Around these central atoms are arranged in the form of a simple geometric polyhedron a certain number of other atoms or molecules. The number of atoms grouped around the central nucleus, Werner called the coordination number. He believed that with a coordination bond there is a common pair of electrons, which one molecule or atom gives to another. Since Werner suggested the existence of compounds that no one had ever observed or synthesized, his theory was distrusted by many famous chemists, who believed that it unnecessarily complicates the understanding of chemical structure and bonds. Therefore, over the next two decades, Werner and his collaborators created new coordination compounds, the existence of which was predicted by his theory. Among the compounds they created were molecules that exhibited optical activity, that is, the ability to deflect polarized light, but did not contain carbon atoms, which were thought to be necessary for the optical activity of the molecules.

In 1911, Werner's synthesis of more than 40 optically active molecules containing no carbon atoms convinced the chemical community of the validity of his theory.

In 1913, Werner was awarded the Nobel Prize in Chemistry "in recognition of his work on the nature of the bonds of atoms in molecules, which made it possible to take a fresh look at the results of previous studies and opened up new opportunities for research work, especially in the field of inorganic chemistry ". According to Theodor Nordström, who represented him on behalf of the Royal Swedish Academy of Sciences, Werner's work "gave impetus to the development of inorganic chemistry", stimulating a revival of interest in this field after it had been neglected for some time.

Structure and stereochemistry

The structure of complex compounds is considered on the basis of the coordination theory proposed in 1893 by the Swiss chemist Alfred Werner, Nobel Prize winner. His scientific activity took place at the University of Zurich. The scientist synthesized many new complex compounds, systematized previously known and newly obtained complex compounds and developed experimental methods for proving their structure.

In accordance with this theory, in complex compounds, a complexing agent, external and internal spheres are distinguished. complexing agent usually is a cation or a neutral atom. inner sphere constitutes a certain number of ions or neutral molecules that are strongly associated with the complexing agent. They are called ligands. The number of ligands determines the coordination number (CN) of the complexing agent. The inner sphere can have a positive, negative, or zero charge.

The rest of the ions that are not located in the inner sphere are located at a farther distance from the central ion, making up external coordination sphere.

If the charge of the ligands compensates for the charge of the complexing agent, then such complex compounds are called neutral or non-electrolyte complexes: they consist only of the complexing agent and ligands of the inner sphere. Such a neutral complex is, for example, .

The nature of the bond between the central ion (atom) and ligands can be twofold. On the one hand, the connection is due to the forces of electrostatic attraction. On the other hand, a bond can form between the central atom and the ligands by the donor-acceptor mechanism, by analogy with the ammonium ion. In many complex compounds, the bond between the central ion (atom) and the ligands is due to both the forces of electrostatic attraction and the bond formed due to the unshared electron pairs of the complexing agent and the free orbitals of the ligands.

Complex compounds with an outer sphere are strong electrolytes and in aqueous solutions dissociate almost completely into a complex ion and ions of the outer sphere.

In exchange reactions, complex ions pass from one compound to another without changing their composition.

The most typical complexing agents are cations of d-elements. Ligands can be:

a) polar molecules - NH 3, H 2 O, CO, NO;
b) simple ions - F - , Cl - , Br - , I - , H + ;
c) complex ions - CN - , SCN - , NO 2 - , OH - .

To describe the relationship between the spatial structure of complex compounds and their physicochemical properties, representations of stereochemistry are used. The stereochemical approach is a convenient technique for representing the properties of a substance in terms of the influence of one or another fragment of the structure of a substance on the property.

The objects of stereochemistry are complex compounds, organic substances, high-molecular synthetic and natural compounds. A. Werner, one of the founders of coordination chemistry, made great efforts to develop inorganic stereochemistry. It is stereochemistry that is central in this theory, which still remains a landmark in coordination chemistry.

Isomerism of coordination compounds

There are two types of isomers:

1) compounds in which the composition of the inner sphere and the structure of the coordinated ligands are identical (geometric, optical, conformational, coordination positions);

2) compounds for which differences are possible in the composition of the inner sphere and the structure of ligands (ionization, hydrate, coordination, ligand).

Spatial (geometric) isomerism

2. Orbitals with lower energy are filled first.

Given these rules, when the number of d-electrons in the complexing agent is from 1 to 3 or 8, 9, 10, they can be arranged in d-orbitals in only one way (in accordance with Hund's rule). With the number of electrons from 4 to 7 in an octahedral complex, it is possible either to occupy orbitals already filled with one electron, or to fill free dγ orbitals of higher energy. In the first case, energy is required to overcome the repulsion between electrons located in the same orbital, in the second case, to move to a higher energy orbital. The distribution of electrons in orbitals depends on the ratio between the energies of splitting (Δ) and pairing of electrons (P). At low values ​​of Δ ("weak field"), the value of Δ can be< Р, тогда электроны займут разные орбитали, а спины их будут параллельны. При этом образуются внешнеорбитальные (высокоспиновые) комплексы, характеризующиеся определённым магнитным моментом µ. Если энергия межэлектронного отталкивания меньше, чем Δ («сильное поле»), то есть Δ >P, pairing of electrons occurs in dε orbitals and the formation of intraorbital (low spin) complexes, the magnetic moment of which µ = 0.

Application

Complex compounds are important for living organisms, so blood hemoglobin forms a complex with oxygen to deliver it to cells, chlorophyll found in plants is a complex.

Complex compounds are widely used in various industries. Chemical methods for extracting metals from ores are associated with the formation of CS. For example, to separate gold from rock, the ore is treated with a sodium cyanide solution in the presence of oxygen. The method of extracting gold from ores using cyanide solutions was proposed in 1843 by the Russian engineer P. Bagration. To obtain pure iron, nickel, cobalt, thermal decomposition of metal carbonyls is used. These compounds are volatile liquids, easily decomposing with the release of the corresponding metals.

Complex compounds have been widely used in analytical chemistry as indicators.

Many CSs have catalytic activity; therefore, they are widely used in inorganic and organic synthesis. Thus, the use of complex compounds is associated with the possibility of obtaining a variety of chemical products: varnishes, paints, metals, photographic materials, catalysts, reliable means for processing and preserving food, etc.

Complex compounds of cyanides are important in electroforming, since it is sometimes impossible to obtain such a strong coating from ordinary salt as when using complexes.

Links

Literature

1. Akhmetov N. S. General and inorganic chemistry. - M.: Higher School, 2003. - 743 p.
2. Glinka N. L. General chemistry. - M.: Higher School, 2003. - 743 p.
3. Kiselev Yu. M. Chemistry of coordination compounds. - M.: Integral-Press, 2008. - 728 p.

Compounds of the type BF 3, CH 4, NH 3, H 2 O, CO 2, etc., in which the element exhibits its usual maximum valence, are called valence-saturated compounds or first order compounds. When first-order compounds interact with each other, higher-order compounds are formed. TO higher order compounds include hydrates, ammoniates, addition products of acids, organic molecules, double salts, and many others. Examples of the formation of complex compounds:

PtCl 4 + 2KCl \u003d PtCl 4 ∙ 2KCl or K 2

CoCl 3 + 6NH 3 \u003d CoCl 3 ∙ 6NH 3 or Cl 3.

A. Werner introduced into chemistry ideas about compounds of a higher order and gave the first definition of the concept of a complex compound. Elements after saturation of ordinary valences are able to show additional valency - coordinating. It is due to the coordination valency that higher-order compounds are formed.

Complex compounds – complex substances that can be isolated central atom(complexing agent) and related molecules and ions - ligands.

The central atom and ligands form complex (inner sphere), which, when writing the formula of a complex compound, is enclosed in square brackets. The number of ligands in the inner sphere is called coordination number. Molecules and ions surrounding the complex form outer sphere. An example of a complex salt of potassium hexacyanoferrate (III) K 3 (the so-called red blood salt).

The central atoms can be transition metal ions or atoms of some non-metals (P, Si). Ligands can be halogen anions (F -, Cl -, Br -, I -), OH -, CN -, CNS -, NO 2 - and others, neutral molecules H 2 O, NH 3, CO, NO, F 2 , Cl 2, Br 2, I 2, hydrazine N 2 H 4, ethylenediamine NH 2 -CH 2 -CH 2 -NH 2, etc.

Coordination valence(CV) or coordination number - the number of places in the inner sphere of the complex that can be occupied by ligands. The coordination number is usually greater than the oxidation state of the complexing agent, depending on the nature of the complexing agent and ligands. Complex compounds with coordination valences of 4, 6, and 2 are more common.

Ligand coordination capacity – the number of places in the inner sphere of the complex occupied by each ligand. For most ligands, the coordination capacity is one, less often 2 (hydrazine, ethylenediamine) and more (EDTA - ethylenediaminetetraacetate).

Complex charge must be numerically equal to the total charge of the outer sphere and opposite in sign, but there are also neutral complexes. The oxidation state of the complexing agent equal and opposite in sign to the algebraic sum of the charges of all other ions.

Systematic names of complex compounds are formed as follows: first, the anion is called in the nominative case, then separately in the genitive case - the cation. The ligands in the complex are listed together in the following order: a) anionic; b) neutral; c) cationic. Anions are listed in the order H - , O 2- , OH - , simple anions, polyatomic anions, organic anions - in alphabetical order. Neutral ligands are named the same as molecules, with the exception of H 2 O (aqua) and NH 3 (ammine); negatively charged ions add the connecting vowel " O". The number of ligands is indicated by prefixes: di-, tri, tetra-, penta-, hexa- etc. The ending for anionic complexes is "- at" or "- new", if the acid is called; there are no typical endings for cationic and neutral complexes.

H - hydrogen tetrachloroaurate (III)

(OH) 2 - tetraamminecopper (II) hydroxide

Cl 4 - hexaammineplatinum (IV) chloride

– tetracarbonyl nickel

– hexacyanoferrate (III) of hexaamminecobalt (III)

Classification of complex compounds based on various principles:

By belonging to a certain class of compounds:

- complex acids– H 2 , H 2 ;

- complex bases- (OH) 2;

- complex salts- Li 3, Cl 2.

By the nature of ligands:

- aquacomplexes(water is the ligand) - SO 4 ∙ H 2 O, [Co (H 2 O) 6] Cl 2;

- ammonia(complexes in which ammonia molecules serve as ligands) - [Сu(NH 3) 4 ]SO 4, Cl;

- acidocomplexes(oxalate, carbonate, cyanide, halide complexes containing anions of various acids as ligands) - K 2, K 4;

- hydroxocomplexes(compounds with OH groups in the form of ligands) - K 3 [Al (OH) 6];

- chelated or cyclic complexes(bi- or polydentate ligand and the central atom form a cycle) - complexes with aminoacetic acid, EDTA; chelates include chlorophyll (complexing agent - magnesium) and hemoglobin (complexing agent - iron).

By the sign of the charge of the complex: cationic, anionic, neutral complexes.

A special group is made up of hypercomplex compounds. In them, the number of ligands exceeds the coordination valency of the complexing agent. So, in the CuSO 4 ∙ 5H 2 O compound, copper has a coordination valence of four and four water molecules are coordinated in the inner sphere, the fifth molecule joins the complex using hydrogen bonds: SO 4 ∙ H 2 O.

Ligands are bound to the central atom donor-acceptor bond. In an aqueous solution, complex compounds can dissociate to form complex ions:

Cl ↔ + + Cl –

To a small extent, there is a dissociation of the inner sphere of the complex:

+ ↔ Ag + + 2NH 3

The measure of the strength of the complex is complex instability constant:

K nest + \u003d C Ag + ∙ C2 NH 3 / C Ag (NH 3) 2] +

Instead of the instability constant, sometimes they use the reciprocal value, called the stability constant:

K mouth \u003d 1 / K nest

In moderately dilute solutions of many complex salts, both complex and simple ions exist. Further dilution can lead to complete decomposition of complex ions.

According to a simple electrostatic model by W. Kossel and A. Magnus, the interaction between a complexing agent and ionic (or polar) ligands obeys the Coulomb law. A stable complex is obtained when the forces of attraction to the core of the complex balance the repulsive forces between the ligands. The strength of the complex increases with an increase in the nuclear charge and a decrease in the radius of the complexing agent and ligands. The electrostatic model is very illustrative, but is unable to explain the existence of complexes with nonpolar ligands and a complexing agent in the zero oxidation state; what determines the magnetic and optical properties of compounds.

A clear way to describe complex compounds is the method of valence bonds (MBS) proposed by Pauling. The method is based on a number of provisions:

The relationship between the complexing agent and the ligands is donor-acceptor. Ligands provide electron pairs, and the core of the complex provides free orbitals. A measure of bond strength is the degree of orbital overlap.

The orbitals of the central atom involved in the formation of bonds undergo hybridization. The type of hybridization is determined by the number, nature, and electronic structure of the ligands. The hybridization of the electron orbitals of the complexing agent determines the geometry of the complex.

Additional strengthening of the complex is due to the fact that, along with σ-bonds, π-bonds can also arise.

The magnetic properties exhibited by the complex are explained on the basis of the occupancy of the orbitals. In the presence of unpaired electrons, the complex is paramagnetic. The pairing of electrons determines the diamagnetism of the complex compound.

MVS is suitable for describing only a limited range of substances and does not explain the optical properties of complex compounds, since does not take into account excited states.

A further development of the electrostatic theory on a quantum mechanical basis is the crystal field theory (TCF). According to TCP, the bond between the core of the complex and the ligands is ionic or ion-dipole. TCP pays the main attention to the consideration of those changes that occur in the complexing agent under the influence of the ligand field (splitting of energy levels). The concept of energy splitting of a complexing agent can be used to explain the magnetic properties and color of complex compounds.

TCP is applicable only to complex compounds in which the complexing agent ( d-element) has free electrons, and does not take into account the partially covalent nature of the complexing agent-ligand bond.

The molecular orbital method (MMO) takes into account the detailed electronic structure of not only the complexing agent, but also the ligands. The complex is considered as a single quantum-mechanical system. The valence electrons of the system are located in multicenter molecular orbitals, covering the nuclei of the complexing agent and all ligands. According to the MMO, the increase in the splitting energy is due to the additional strengthening of the covalent bond due to π-bonding.

Fundamentals of modern coordination theory were outlined at the end of the last century by the Swiss chemist Alfred Werner, who generalized into a single system all the experimental material accumulated by that time on complex compounds. They introduced the concept of central atom (complexing agent) and his coordination number, internal And external sphere complex connection, isomerism complex compounds, attempts have been made to explain the nature of the chemical bond in the complexes.

All the main provisions coordination theory Werner are still in use today. The exception is his doctrine of the nature of the chemical bond, which is now only of historical interest.

The formation of a complex ion or a neutral complex can be thought of as a reversible reaction of a general type:

M+ n L

where M is a neutral atom, a positively or negatively charged conditional ion that unites (coordinates) around itself other atoms, ions or molecules L. The atom M is called complexing agent or central atom.

In complex ions 2+ , 2

- , 4 - , - complexing agents are copper(II), silicon(IV), iron(II), boron(III).
Most often, the complexing agent is an atom of the element in a positive oxidation state.
Negative conditional ions (i.e. atoms in negative oxidation states) play the role of complexing agents relatively rarely. This, for example, is a nitrogen atom (-III) in an ammonium cation +, etc.

The complexing atom may have null degree of oxidation. Thus, carbonyl complexes of nickel and iron, having the composition and , contain nickel(0) and iron(0) atoms.

complexing agent (highlighted blue color) can participate in the reactions of obtaining complexes, as being a monatomic ion, for example:

Ag+ + 2 NH 3 [ Ag(NH 3) 2] +;
Ag+ +2 CN - [Ag(CN) 2 ]

-

and being part of the molecule:

Si F 4 + 2 F

- [Si F 6 ] 2- ;

I 2+I

- [I(I) 2 ] - ;

P H 3 + H + [ P H4]+;

B F 3 + NH 3 [ B(NH3)F3]

A complex particle may contain two or more complexing atoms. In this case, one speaks of .

Complex connection may include several complex ions, each containing its own complexing agent.
For example, in a mononuclear complex compound of composition (SO 4) 2 complexing agents are K I and Al III, and in - Cu II and Pt IV.

In a complex ion or neutral complex, ions, atoms, or simple molecules (L) are coordinated around the complexing agent. All these particles that have chemical bonds with the complexing agent are called ligands(from Latin " ligare"- bind). In complex ions 2

- and 4 - Cl ions are ligands- and CN - , and in the neutral complex, the ligands are NH 3 molecules and NCS - ions.

Ligands, as a rule, are not bound to each other, and repulsive forces act between them. In some cases, intermolecular interaction of ligands is observed with the formation hydrogen bonds.

Ligands can be various inorganic and organic ions And molecules. The most important ligands are CN ions

- , F - , Cl - , Br - , I - , NO 2 - , OH - , SO 3 S 2- , C 2 O 4 2- , CO 3 2- , molecules H 2 O, NH 3, CO, urea (NH 2) 2CO, organic compounds - ethylenediamine NH 2 CH 2 CH 2 NH 2, a -aminoacetic acid NH 2 CH 2COOH and ethylenediaminetetraacetic acid (EDTA):

Most often, the ligand is bound to the complexing agent through one of its atoms. one two-center chemical bond. Such ligands are called monodentate. Monodentate ligands include all halide ions, cyanide ion, ammonia, water, and others.

Some common ligands like water molecules H 2

O, hydroxide ion OH - , thiocyanate ion NCS-, amide ion NH 2 - , carbon monoxide CO in complexes predominantly monodentate, although in some cases (in structures) they become bidentate.

There are a number of ligands, which in complexes are almost always bidentate. These are ethylenediamine, carbonate ion, oxalate ion, etc. Each molecule or ion of the bidentate ligand forms two chemical bonds with the complexing agent in accordance with the features of its structure:

For example, in the complex compound NO 3

bidentate ligand - CO 3 2 ion- - forms two bonds with a complexing agent, a conditional Co(III) ion, and each NH 3 ligand molecule– only one connection:

An example of a hexadentate ligand is the anion of ethylenediaminetetraacetic acid:

Polydentate ligands can act as

bridge ligands that link two or more central atoms.

The most important characteristic of a complexing agent is the number of chemical bonds that it forms with ligands, or coordination number (KCH). This characteristic of the complexing agent is determined mainly by the structure of its electron shell and is determined by valence possibilities central atom or conditional complexing ion ().

When the complexing agent coordinates monodentate ligands, then the coordination number is equal to the number of attached ligands. And the number of compounds attached to the complexing agent polydentate ligands is always less than the value of the coordination number.

The value of the coordination number complexing agent depends on its nature, degree of oxidation, nature of ligands, and conditions (temperature, nature of solvent, concentration of complexing agent and ligands, etc.) under which the complexation reaction proceeds. The CN value can vary in various complex compounds from 2 to 8 and even higher. The most common coordination numbers are 4 and 6.

Between the values ​​of the coordination number and the degree of oxidation of the complexing element there is certain dependency. Yes, for complexing elements, having an oxidation state + I (Ag I, Cu I, Au I, I I

etc.) the most typical coordination number is 2 - for example, in complexes of the + type, - , - .

With oxidation state +II (Zn

II , Pt II , Pd II , Cu II etc.) often form complexes in which they exhibit a coordination number of 4, such as 2+, 2- , 0 , 2

- , 2+ .

IN aqua complexes the coordination number of the complexing agent in the +II oxidation state is most often 6: 2+ , 2+ , 2+ .

Complexing elements, with oxidation states + III and + IV (Pt IV, Al III, Co III, Cr III, Fe III), have in complexes, as a rule, CN 6.
For example, 3+ , 3- , 2 - , 3 - , 3 - .

Known complexing agents that have practically constant coordination number in complexes of different types. These are cobalt(III), chromium(III) or platinum(IV) with a c.n. of 6 and boron(III), platinum(II), palladium(II), gold(III) with a c.n. of 4. Nevertheless, most complexing agents have a variable coordination number. For example, for aluminum(III), CN 4 and CN 6 are possible in the complexes

- And - .

Coordination numbers 3, 5, 7, 8 and 9 are relatively rare. There are only a few compounds in which the CN is 12 - for example, such as K 9 .

If a complex ion or neutral complex contains two or more complexing agents, then this complex is called multi-core. Among the multinuclear complexes, there are bridging,

cluster and multinuclear complexes mixed type.

Atoms of the complexing agent can be bonded to each other via bridging ligands, whose functions are performed by ions OH -, Cl -, NH 2 -, O 2 2-, SO 4 2- and some others.
So, in the complex compound (NH 4) 2 bridge serve bidentate hydroxide ligands :

Cast bridging ligand a polydentate ligand having several donor atoms can act (for example, NCS - with N and S atoms capable of participating in the formation of bonds by the donor-acceptor mechanism), or a ligand with several electron pairs at the same atom (for example, Cl - or OH -).

In the case when the atoms of the complexing agent are directly linked, the multinuclear complex is referred to as cluster type.
Thus, the cluster is the complex anion 2

- :

in which the Re-Re quadruple bond is realized: one σ-bond, two π-bonds, and one δ-bond. A particularly large number of cluster complexes are found among the derivatives d-elements.

Multinuclear complexes mixed type contain as link complexing agent–complexing agent, and bridging ligands.
An example of a mixed-type complex is the cobalt carbonyl complex with the composition , having the following structure:

Here there is a single bond Co - Co and two bidentate carbonyl ligands CO, which carry out the bridge connection of complexing atoms.

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Repeat:

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Compounds are called complex, in the nodes of the crystals of which there are complexes (complex ions) capable of independent existence.

The value of complex compounds for various fields of technology is very high. The ability of substances to form complex compounds is used to develop effective methods for obtaining chemically pure metals from ores, rare metals, ultrapure semiconductor materials, catalysts, dyes, drugs, natural and waste water purification, scale dissolution in steam generators, etc.

The first complex compounds were synthesized in the middle of the 19th century. The founder of the theory of complex compounds was the Swiss scientist Werner, who developed in 1893 coordination theory . A great contribution to the chemistry of complex compounds was made by Russian scientists L.A. Chugaev, I.I. Chernyaev and their students.

Structure of complex compounds:

1. In each complex compound, inner and outer spheres. The inner sphere is called the complex. When writing chemical formulas of complex compounds, the inner sphere is enclosed in square brackets. For example, in complex compounds a) K 2 [BeF 4], b) Cl 2, the inner sphere is made up of groups of atoms - complexes a) [BeF 4] 2- and b) 2+, and the outer sphere is made up, respectively, by ions a) 2K + and b) 2Cl - .

2. In the molecule of any complex compound, one of the ions, usually positively charged, or an atom of the internal environment occupies a central place and is called complexing agent. In the formula of a complex (inner sphere), the complexing agent is indicated first. In the examples given, these are ions a) Be 2+ and b) Zn 2+.

The complexing agents are atoms or more often metal ions related to p-, d-, f- elements and having a sufficient number of free orbitals (Cu 2+, Pt 2+, Pt 4+, Ag +, Zn 2+, Al 3+, etc. ).

3. Around the complexing agent is located (or, as they say, coordinated) a certain number of oppositely charged ions or electrically neutral molecules, called ligands(or addends). In this case, these are a) F ions - and b) NH 3 molecules.

Anions F - , OH - , CN - , CNS - , NO 2 - , CO 3 2- , C 2 O 4 2- , etc., neutral molecules H 2 O, NH 3 , CO, NO and etc.

The number of coordination sites occupied by ligands around the complexing agent (in the simplest cases, the number of ligands surrounding the complexing agent) is called coordination number (c.h.) of the complexing agent. The coordination numbers of various complexing agents range from 2 to 12.

The most characteristic coordination numbers in solutions and the charge of the central ion (complexing agent) are compared below:

Note: The most common coordination numbers are underlined when two different types of coordination are possible.

In the considered examples, the coordination numbers of the complexing agents are: a) k.ch. (Be 2+) = 4, b) c.h. (Zn 2+) = 4.

B. Then they call the numbers and names of neutral ligands:

B. The last name is the complexing agent in the genitive case, indicating the degree of its oxidation (in brackets in Roman numerals after the name of the complexing agent).

For example, Cl is chlorotriammineplatinum (II) chloride.

If the metal forms an ion with one oxidation state, then it may not be included in the name of the complex. For example, Cl 2 is tetraamminzinc dichloride.

2. Name of the complex anion formed in a similar way, with the addition of the suffix "at" to the root of the Latin name of the complexing agent (for example, ferrate, nickelate, chromate, cobaltate, cuprate, etc.). For example:

K 2 - potassium hexachloroplatinate (IV);

Ba 2 - barium tetrarodanodiammine chromate (III);

K 3 - hexacyanoferrate (III) potassium;

K 2 - potassium tetrafluoroberyllate.

3. Names of neutral complex particles are formed in the same way as cations, but the complexing agent is called in the nominative case, and the degree of its oxidation is not indicated, because it is determined by the electroneutrality of the complex. For example:

Dichlorodiammineplatinum;

Tetracarbonyl nickel.

Classification of complex compounds. Complex compounds are very diverse in structure and properties. Their classification systems are based on various principles:

1. According to the nature of the electric charge, cationic, anionic and neutral complexes are distinguished.

A complex with a positive charge is called cationic, for example 2+, with a negative charge - anionic, for example 2-, with a zero charge - neutral, for example.

2. The types of ligands are:

a) acids, for example:

H is hydrogen tetrachloroaurate (III);

H 2 - hexachloroplatinate (IV) hydrogen;

b) grounds, for example:

(OH) 2 - tetraammine copper (II) hydroxide;

OH - diamminesilver hydroxide;

c) salt, for example:

K 3 - potassium hexahydroxoaluminate;

Cl 3 - hexaaquachromium (III) chloride;

d) non-electrolytes, for example, dichlorodiammineplatinum.

Formation of chemical bonds in complex compounds. To explain the formation and properties of complex compounds, a number of theories are currently used:

1) method of valence bonds (MVS);

2) the theory of the crystal field;

3) the method of molecular orbitals.

According to the MVS during the formation of complexes between the complexing agent and ligands, a covalent bond arises along donor-acceptor mechanism . Complexing agents have vacant orbitals; play the role of acceptors. As a rule, various vacant orbitals of the complexing agent are involved in the formation of bonds; therefore, their hybridization occurs. Ligands have lone pairs of electrons and play the role of donors in the donor-acceptor mechanism of covalent bond formation.

For example, consider the formation of the 2+ complex. Electronic formulas of valence electrons:

Zn atom - 3d 10 4s 2 ;

Zinc ion complexing agent

Zn 2+ - 3d 10 4s 0

As can be seen, the zinc ion at the outer electronic level has four vacant atomic orbitals close in energy (one 4s and three 4p), which will undergo sp 3 hybridization; the Zn 2+ ion, as a complexing agent, has c.h.=4.

When a zinc ion interacts with ammonia molecules, the nitrogen atoms of which have lone pairs of electrons (: NH 3), a complex is formed:

The spatial structure of the complex is determined by the type of hybridization of the atomic orbitals of the complexing agent (in this case, a tetrahedron). The coordination number depends on the number of vacant orbitals of the complexing agent.

In the formation of donor-acceptor bonds in complexes, not only s- and p-orbitals, but also d-orbitals can be used. In these cases, hybridization occurs with the participation of d-orbitals. The table below shows some types of hybridization and their corresponding spatial structures:

Thus, the MVS makes it possible to predict the composition and structure of the complex. However, this method cannot explain such properties of complexes as strength, color, and magnetic properties. The above properties of complex compounds are described by the crystal field theory.

Dissociation of complex compounds in solutions. The inner and outer spheres of a complex compound differ greatly in stability.

Particles located in the outer sphere are associated with the complex ion mainly by electrostatic forces (ionic bond) and are easily split off in an aqueous solution, like ions of strong electrolytes.

The dissociation (decay) of a complex compound into ions of the outer sphere and a complex ion (complex) is called primary. It proceeds almost completely, to the end, according to the type of dissociation of strong electrolytes.

For example, the process of primary dissociation during the dissolution of potassium tetrafluoroberyllate can be written according to the scheme:

K 2 [BeF 4] = 2K + + [BeF 4] 2-.

Ligands, located in the inner sphere of the complex compound, are associated with the complexing agent by strong covalent bonds formed according to the donor-acceptor mechanism, and the dissociation of complex ions in solution occurs, as a rule, to a small extent by the type of dissociation of weak electrolytes, i.e. reversible until equilibrium is established. The reversible decay of the inner sphere of a complex compound is called secondary dissociation. For example, the tetrafluoroberyllate ion only partially dissociates, which is expressed by the equation

[BeF 4 ] 2- D Be 2+ + 4F - (secondary dissociation equation).

The dissociation of a complex as a reversible process is characterized by an equilibrium constant called the instability constant of the complex K n.

For the example in question:

K n - tabular (reference) value. The instability constants, whose expressions include the concentrations of ions and molecules, are called concentration constants. More stringent and independent of the composition and ionic strength of the solution are K n, containing instead of the concentration of the activity of ions and molecules.

The Kn values ​​of various complexes vary widely and can serve as a measure of their stability. The more stable the complex ion, the lower its instability constant.

Thus, among similar compounds with different values ​​of instability constants

the most stable complex is , and the least stable is .

Like any equilibrium constant, instability constant depends only on the nature of the complex ion, complexing agent and ligands, solvent, as well as on temperature and does not depend on the concentration (activity) of substances in solution.

The greater the charges of the complexing agent and ligands and the smaller their radii, the higher the stability of the complexes . The strength of the complex ions formed by the metals of the secondary subgroups is higher than the strength of the ions formed by the metals of the main subgroups.

The process of decomposition of complex ions in solution proceeds in many stages, with successive elimination of ligands. For example, the dissociation of the copper (II) 2+ ammonia ion occurs in four steps, corresponding to the separation of one, two, three, and four ammonia molecules:

For a comparative assessment of the strength of various complex ions, not the dissociation constant of individual steps is used, but the general instability constant of the entire complex, which is determined by multiplying the corresponding stepwise dissociation constants. For example, the instability constant of the 2+ ion will be equal to:

K H \u003d K D1 K D2 K D3 K D4 \u003d 2.1 10 -13.

To characterize the strength (stability) of complexes, the reciprocal of the instability constant is also used, it is called the stability constant (Kst) or the complex formation constant:

The equilibrium of dissociation of a complex ion can be shifted by an excess of ligands in the direction of its formation, and a decrease in the concentration of one of the dissociation products, on the contrary, can lead to the complete destruction of the complex.

Qualitative chemical reactions usually detect only outer sphere ions or complex ions. Although everything depends on the solubility product (SP) of the salt, the formation of which would proceed with the addition of appropriate solutions in qualitative reactions. This can be seen from the following reactions. If a solution containing a complex ion + is acted upon by a solution of any chloride, then no precipitate is formed, although a precipitate of silver chloride is released from solutions of ordinary silver salts when chlorides are added.

Obviously, the concentration of silver ions in the solution is too low, so that when even an excess of chloride ions is introduced into it, it would be possible to achieve the value of the solubility product of silver chloride (PR AgCl = 1.8 10 -10). However, after the addition of the potassium iodide complex to the solution, a precipitate of silver iodide precipitates. This proves that silver ions are still present in the solution. No matter how small their concentration, but it turns out to be sufficient for the formation of a precipitate, because. PR AgI \u003d 1 10 -16, i.e. much less than that of silver chloride. In the same way, under the action of a solution of H 2 S, a precipitate of silver sulfide Ag 2 S is obtained, the solubility product of which is 10 -51.

The ion-molecular equations of the ongoing reactions have the form:

I - D AgI↓ + 2NH 3

2 + + H 2 S D Ag 2 S↓ + 2NH 3 + 2NH 4 + .

Complex compounds with an unstable inner sphere are called double salts. They are designated differently, namely, as compounds of molecules. For example: CaCO 3 Na 2 CO 3; CuCl 2 ·KCl; KCl·MgCl 2 ; 2NaCl · CoCl 2 . double salts can be considered as compounds in the crystal lattice sites of which there are identical anions, but different cations; chemical bonds in these compounds are predominantly ionic in nature and therefore in aqueous solutions they dissociate almost completely into separate ions. If, for example, potassium chloride and copper (II) chloride are dissolved in water, then dissociation occurs according to the type of strong electrolyte:

CuCl 2 KCl \u003d Cu 2+ + 3Cl - + K +.

All ions formed in a double salt solution can be detected using appropriate qualitative reactions.

Reactions in solutions of complex compounds. The equilibrium shift in exchange reactions in electrolyte solutions involving complex ions is determined by the same rules as in solutions of simple (non-complex) electrolytes, namely: the equilibrium shifts in the direction of the most complete binding of ions (complexing agent, ligands, ions of the outer sphere), leading to to the formation of insoluble, poorly soluble substances or weak electrolytes.

In this regard, in solutions of complex compounds, reactions are possible:

1) exchange of ions of the outer sphere, in which the composition of the complex ion remains constant;

2) intrasphere exchange.

The first type of reaction is realized in those cases when it leads to the formation of insoluble and poorly soluble compounds. An example is the interaction of K 4 and K 3, respectively, with the cations Fe 3+ and Fe 2+, which gives a precipitate of Prussian blue Fe 4 3 and turnbull blue Fe 3 2:

3 4- + 4Fe 3+ = Fe 4 3 ↓,

Prussian blue

2 3- + 3Fe 2+ = Fe 3 2 ↓.

turnbull blue

Reactions of the second type are possible in those cases when this leads to the formation of a more stable complex, i.e. with a lower value of K n, For example:

2S 2 O 3 2- D 3- + 2NH 3.

K n: 9.3 10 -8 1 10 -13

At close values ​​of Kn, the possibility of such a process is determined by the excess of the competing ligand.

For complex compounds, redox reactions are also possible, which take place without changing the atomic composition of the complex ion, but with a change in its charge, for example:

2K 3 + H 2 O 2 + 2KOH \u003d 2 K 4 + O 2 + 2H 2 O.