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 divides 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.
Preparation of complex compounds different ways 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 indicated first - 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
History
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 "On the Theory of Affinity and Valence", published in 1891, 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 therefore 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 nonelectrolyte 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, with the use of complex compounds, the possibility of obtaining a variety of chemical products is associated: 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
- Akhmetov N. S. General and inorganic chemistry. - M.: high school, 2003. - 743 p.
- Glinka N. L. General chemistry. - M.: Higher School, 2003. - 743 p.
- Kiselev Yu. M. Chemistry of coordination compounds. - M.: Integral-Press, 2008. - 728 p.
Complex compounds
Lesson-lecture Grade 11
The lesson submitted for the competition “I'm going to the lesson”, I spend in the 11th biological and chemical class, where 4 hours a week are allotted for studying chemistry.
I took the topic “Complex compounds”, firstly, because this group of substances has exclusively great importance in nature; second, many USE assignments include the concept of complex compounds; thirdly, students from this class choose professions related to chemistry and will meet with a group of complex compounds in the future.
Target. Form the concept of the composition, classification, structure and basic nomenclature of complex compounds; consider their chemical properties and show the meaning; expand students' understanding of the diversity of substances.
Equipment. Samples of complex compounds.
Lesson Plan
I. Organizing time.
II. Learning new material (lecture).
III. Summing up and setting homework.
Lecture plan
1. Variety of substances.
2. Coordination theory of A. Werner.
3. Structure of complex compounds.
4. Classification of complex compounds.
5. The nature of the chemical bond in complex compounds.
6. Nomenclature of complex compounds.
7. Chemical properties complex compounds.
8. The value of complex compounds.
DURING THE CLASSES
I. Organizational moment
II. Learning new material
Variety of substances
The world of substances is diverse, and we are already familiar with the group of substances that belong to complex compounds. These substances have been studied since the 19th century, but it was difficult to understand their structure from the standpoint of the existing ideas about valency.
A. Werner's coordination theory
In 1893, the Swiss inorganic chemist Alfred Werner (1866–1919) formulated a theory that made it possible to understand the structure and some properties of complex compounds and called coordination theory*. Therefore, complex compounds are often called coordination compounds.
Compounds, which include complex ions that exist both in a crystal and in solution, are called complex, or coordination.
The structure of complex compounds
According to Werner's theory, the central position in complex compounds is usually occupied by a metal ion, which is called the central ion, or complexing agent.
Complexing agent - a particle (atom, ion or molecule) that coordinates (situates) around itself other ions or molecules.
The complexing agent usually has a positive charge, is d-element, exhibits amphoteric properties, has a coordination number of 4 or 6. Molecules or acid residues - ligands (addends) are located (coordinate) around the complexing agent.
Ligands - particles (molecules and ions) coordinated by the complexing agent and having direct chemical bonds with it (for example, ions: Cl - , I - , NO 3 - , OH - ; neutral molecules: NH 3 , H 2 O, CO ).
The ligands are not bound to each other, since repulsive forces act between them. When molecules are ligands, molecular interaction is possible between them. The coordination of ligands around the complexing agent is feature complex compounds (Fig. 1).
Coordination number - is the number of chemical bonds that the complexing agent forms with the ligands.
 |
Rice. 2. Tetrahedral structure of the ion - |
The value of the coordination number of the complexing agent depends on its nature, degree of oxidation, nature of the ligands, and conditions (temperature, concentration) under which the complexation reaction proceeds. The coordination number can have values from 2 to 12. The most common are the coordination numbers 4 and 6. For the coordination number 4, the structure of complex particles can be tetrahedral (Fig. 2) and in the form of a flat square (Fig. 3). Complex compounds with a coordination number of 6 have an octahedral structure of 3– (Fig. 4).
 |
Rice. 4. Ion 3 - octahedral structure |
The complexing agent and its surrounding ligands constitute the interior of the complex. A particle consisting of a complexing agent and surrounding ligands is called a complex ion. When depicting complex compounds, the inner sphere (complex ion) is limited by square brackets. The remaining components of the complex compound are located in external sphere(Fig. 5).
The total charge of the ions of the outer sphere must be equal in value and opposite in sign to the charge of the complex ion:
Classification of complex compounds
A large variety of complex compounds and their properties does not allow creating a unified classification. However, substances can be grouped according to some individual characteristics.
1) By composition.
2) According to the type of coordinated ligands.
but) Aquacomplexes- these are complex cations in which H 2 O molecules are ligands. They are formed by metal cations with an oxidation state of +2 or more, and the ability to form aqua complexes in metals of one group of the periodic system decreases from top to bottom.
Examples of aqua complexes:
Cl 3 , (NO 3) 3 .
b) Hydroxocomplexes are complex anions in which the ligands are hydroxide ions OH - . Complexing agents are metals prone to the manifestation of amphoteric properties - Be, Zn, Al, Cr.
For example: Na, Ba.
in) Ammonia are complex cations in which NH 3 molecules are ligands. Complexing agents are d-elements.
For example: SO 4 , Cl.
G) acidocomplexes are complex anions in which the ligands are anions of inorganic and organic acids.
For example: K 3 , Na 2 , K 4 .
3) By the charge of the inner sphere.
The nature of the chemical bond in complex compounds
In the inner sphere, there are covalent bonds between the complexing agent and ligands, which are also formed by the donor-acceptor mechanism. For the formation of such bonds, the presence of free orbitals in some particles (available in the complexing agent) and unshared electron pairs in other particles (ligands) is necessary. The role of the donor (supplier of electrons) is played by the ligand, and the acceptor that accepts electrons is the complexing agent. The donor-acceptor bond arises as a result of the overlapping of the free valence orbitals of the complexing agent with the filled donor orbitals.
There is an ionic bond between the outer and inner spheres. Let's take an example.
The electronic structure of the beryllium atom:
The electronic structure of the beryllium atom in an excited state:
The electronic structure of the beryllium atom in the 2– complex ion:
Dotted arrows show fluorine electrons; two of the four bonds are formed by the donor-acceptor mechanism. In this case, the Be atom is an acceptor, and fluorine ions are donors, their free electron pairs fill hybridized orbitals ( sp 3 - hybridization).
Nomenclature of complex compounds
The most widespread is the nomenclature recommended by IUPAC. Name complex anion begins with the designation of the composition of the inner sphere: the number of ligands is indicated by Greek numerals: 2-di, 3-three, 4-tetra, 5-penta, 6-hexa, etc., followed by the names of the ligands, to which the connecting vowel “o” is added »: Cl - - chloro-, CN - - cyano-, OH - - hydroxo-, etc. If the complexing agent has a variable oxidation state, then its oxidation state is indicated in brackets in Roman numerals, and its name with the suffix -at: Zn - zinc at, Fe – ferr at(III), Au - aur at(III). The last name is the cation of the outer sphere in the genitive case.
K 3 - potassium hexacyanoferrate (III),
K 4 - potassium hexacyanoferrate (II),
K 2 - potassium tetrahydroxozincate.
Names of compounds containing complex cation, are built from the names of the anions of the external environment, after which the number of ligands is indicated, the Latin name of the ligand is given (ammonia molecule NH 3 - ammine, water molecule H 2 O - aqua from the Latin name of water) and the Russian name of the complexing element; the Roman numeral in parentheses indicates the degree of oxidation of the complexing element, if it is variable. For example:
SO 4 - tetraammine copper (II) sulfate,
Cl 3 - hexaaqua aluminum chloride.
Chemical properties of complex compounds
1. In solution, complex compounds behave like strong electrolytes; completely dissociate into cations and anions:
Cl 2 \u003d Pt (NH 3) 4] 2+ + 2Cl -,
K 2 \u003d 2K + + 2–.
Dissociation of this type is called primary.
Secondary dissociation is associated with the removal of ligands from the inner sphere of the complex ion:
2– PtCl 3 – + Cl – .
Secondary dissociation occurs in steps: complex ions ( 2–) are weak electrolytes.
2. Under the action of strong acids, hydroxo complexes are destroyed, for example:
a) with a lack of acid
Na 3 + 3HCl \u003d 3NaCl + Al (OH) 3 + 3H 2 O;
b) with an excess of acid
Na 3 + 6HCl \u003d 3NaCl + AlCl 3 + 6H 2 O.
3. Heating (thermolysis) of all ammoniates leads to their decomposition, for example:
SO 4 CuSO 4 + 4NH 3.
The value of complex compounds
Coordination compounds are extremely important in nature. Suffice it to say that almost all enzymes, many hormones, drugs, biologically active substances are complex compounds. For example, blood hemoglobin, due to which oxygen is transferred from the lungs to tissue cells, is a complex compound containing iron (Fig. 6), and chlorophyll, responsible for photosynthesis in plants, is a complex magnesium compound (Fig. 7).
A significant part of natural minerals, including polymetallic ores and silicates, is also composed of coordination compounds. Moreover, chemical methods for extracting metals from ores, in particular copper, tungsten, silver, aluminum, platinum, iron, gold and others, are also associated with the formation of easily soluble, low-melting or volatile complexes. For example: Na 3 - cryolite, KNa 3 4 - nepheline (minerals, complex compounds containing aluminum).
The modern chemical industry widely uses coordination compounds as catalysts in the synthesis of macromolecular compounds, in the chemical processing of oil, and in the production of acids.
III. Summing up and staging homework
Homework.
1) Prepare for a lecture for a practical lesson on the topic: “Complex compounds”.
2) Give a written description of the following complex compounds by structure and classify according to their characteristics:
K 3, (NO 3) 3, Na 2, OH.
3) Write the reaction equations with which you can carry out transformations:
* For the discovery of this new field of science, A. Werner was awarded the Nobel Prize in 1913.
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 production of chemically pure metals from ores, rare metals, ultrapure semiconductor materials, catalysts, dyes, medicines, natural and waste water treatment, 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 are possible. various types coordination.
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, they use not the dissociation constant of individual steps, 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.
With the help of quality chemical reactions usually only outer sphere ions or complex ions are found. 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 individual 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 with the participation of 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), 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, taking 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.
C 5. Ligands directly associated with the complexing agent form together with it internal (coordination) sphere of the complex. Thus, in the complex cation 2+, the inner sphere is formed by the atom of the complexing agent, copper(II), and ammonia molecules directly bound to it. The inner sphere is denoted by square brackets: 3 , 2 , 2 . Depending on the ratio of the total charge of the ligands and the complexing agent, the inner sphere may have a positive charge, for example, 3+ , either negative, for example, 3 , or zero charge, for example, as for 0 .
Ions that neutralize the charge of the inner sphere, but are not covalently bound to the complexing agent, form the outer sphere of the complex compound. For example, in the Cl 2 complex compound, two Cl ions are in the outer sphere:
Outer sphere Cl ions are located at a greater distance from the complexing agent than NH 3 molecules, in other words, the Zn - Cl distance is greater than the length of the Zn - N chemical bond. Moreover, the chemical bond of the complex cation 2+ and chloride ions Cl - has an ionic character, while ammonia NH 3 entering the inner sphere form covalent bonds with the complexing agent Zn(II) according to the donor-acceptor mechanism (the donor of unshared electron pairs are nitrogen atoms in NH 3). So the difference between inner sphere ligands And outer sphere ions very significant.
In (OH) 2 and K 2 the outer sphere ions are OH and K + ions, respectively. It is quite clear that in neutral complexes 0 and 0 outer sphere missing.
C 5. Usually the outer sphere is made up of simple monatomic or polyatomic ions. However, there are cases when the CS consists from two or more inner spheres, performing the functions of the cationic and anionic parts of the compound. Here each of the internal spheres is external to the other. For example, in compounds and 2, formally, the functions of outer-sphere ions can be performed by:
complex cations 2+ and 2+,
complex anions 2 and 4
1.6. Multinuclear complexes
C 8. If the 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 (2 bonds) hydroxide ligands:
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 quadruple bond Re-Re: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 bridge the complexing atoms.
