Complex compounds: nomenclature and classification

The largest and most diverse among inorganic substances is the class of complex compounds. It can include a group of organometallic substances, such as chlorophyll and hemoglobin. It is these compounds that are the bridge that connects inorganic and organic chemistry into a single science. The role of complex substances in the development of knowledge in the field of analytical chemistry and crystal chemistry, in the study of the most important biological processes: photosynthesis, internal (cellular) respiration is invaluable.

In this article we will study the structure and nomenclature of complex compounds, as well as the basic principles of their classification.

A. Werner's coordination theory

At the end of the 20th century the Swiss scientist A. Werner proved that in the molecule of any complex substance there are several structures that were respectively called the central ion, ligands (addends) and the outer coordination sphere. In order to clarify the classification and nomenclature of complex compounds, we will analyze these concepts in more detail. So, A. Werner was proved the presence in the molecule of an ion (usually positively charged), occupying a central position. It became known as a complexing agent, a central ion, or an atom. Near it can be located both neutral molecules, called ligands, and negatively charged anion particles, which form the internal coordination sphere of matter. All the remaining particles, not included in it, form the outer shell of the molecule.

Thus, in the sodium cuprite formula Na ₂ [Cu (OH) ₄ ], the central copper atom in the oxidation state of +2 and four hydroxo groups constitute the inner sphere, and the sodium ions are located some distance from the central atom in the outer sphere.

Methods for determining coordination formulas and names of substances

Up to the present time, A. Werner's theory remains the main theoretical base on which complex complex compounds are studied. Nomenclature, that is, the names of these substances, are determined by the rules adopted by the International Society of Theoretical and Applied Chemistry.

Let us give some examples of formulas of substances in which the complexing agent is represented by the platinum-K ₂ [PtCl ₆ ] atom or NH ₃ - [Ag (NH ₃ ) ₂ ] Cl. As it turned out, the formulas can be derived with the help of the following practical methods: by double exchange reactions, by the molar electrical conductivity of solutions, by the X-ray diffraction method. Let's consider these methods in more detail.

As the structure of complex compounds of platinum

Substances of this group are characterized by the presence in the molecule of the central atom of platinum. If a solution of silver nitrate is applied to the PtCl ₄ × 6NH ₃ compound , then all the chlorine present in the substance binds to the metal atoms and white flakes of AgCl are formed. This means that all the chlorine anions were in the outer coordination sphere, whereas the ammonia molecules were bound to the central platinum atom and together with it formed an inner sphere.

Hence, the coordination formula for the substance will be written in the following form: [Pt (NH ₃ ) ₆ ] Cl ₄ and named platinum hexamine chloride. Using the X-ray diffraction method, chemists also studied other complex compounds, the nomenclature of which will be established by us in the next section.

Crystalline compounds of chromium

The structure of the substances of this group was determined by the physical process of X-ray diffraction underlying the X-ray diffraction analysis. Passing through the crystal lattice, electromagnetic waves are scattered by the electrons of the substance under investigation. This makes it possible to establish very precisely which groups of atoms are at the sites of the crystal lattice. For chromium-containing crystals, a corresponding nomenclature of complex compounds was created. Examples of the names of isomeric hydrates of trivalent chromium salts made using the X-ray diffraction method are as follows: tetraacvadichlorochrome (III) chloride, pentaacochlorochrome (III) chloride.

It was found that in these substances the chromium atom is associated with six different addends. How do you determine this indicator and what factor affects the coordination number?

As the central atom is associated with ligands

To answer the question raised above, we recall that in the immediate vicinity of the complexing agent there are several structures called addends or ligands. Their total number determines the coordination number. According to the theory of A. Werner, the production, classification and nomenclature of complex compounds directly depend on this indicator. It is correlatively related to the degree of oxidation of the central atom. In the compounds of platinum, chromium, iron, the coordination number is usually six; If the complexing agent is represented by copper or zinc atoms - four, if the central atom is silver or copper - to two.

Types of complex compounds

In chemistry, both the main classes and the transitional series of substances between them are distinguished. Complex compounds considered in the previous subheadings, whose nomenclature indicates the presence of water molecules in their structure, refer to aquacomplexes. To ammonia include substances containing neutral ammonia particles, for example triiodium triammotropium. The class of chelate compounds is unique in the structure of molecules. Their name comes from the biological term chelicera - the so-called claws of decapod crustaceans. These substances contain addends, the spatial configuration of which covers the complexing agent, like claws. Such compounds include oxalate complex of ferric iron, ethylenediamine complex of platinum with oxidation state +4, salts of aminoacetic acid, which include rhodium, platinum or copper ions.

Rules for the design of complex compound names

The most common control question in chemistry assignments in the high school course is: call the complex compounds on the IUPAC nomenclature. On a concrete example, let's analyze the algorithm for the formulation of the name of a substance having the formula: (NH ₄ ) ₂ [Pt (OH) ₂ Cl ₄ ].

1. The name begins with the definition of the composition of the internal coordination sphere. It contains anions of hydroxyl groups and chlorine. To their names we add the ending -o. We get: dihydroxo, tetrachloro.
2. Now we find the complexing agent, using the Latin name for its designation, and add to it the suffix -at, in parentheses we indicate its oxidation state: platinum (IV).
3. Having finished with the designation of the inner sphere, proceed to the outer part. Let's call it cations: in our example it will be ammonium ions.

As a result, the substance will have a name in which all the above structures are listed.

Application of complex compounds

At the beginning of the article we called the most important representatives of organometallic substances, such as hemoglobin, chlorophyll, vitamins. They play a leading role in the metabolism. Complex compounds are widely used in the technological cycles of smelting of ferrous and non-ferrous metals. Important role in metallurgy is played by carbonyls - special complex compounds whose nomenclature indicates the presence in their molecules of carbon monoxide CO in the form of an addendum. These compounds decompose on heating and restore metals such as nickel, iron, cobalt from their ores. Most complex compounds are also used as catalysts in reactions for the production of varnishes, paints and plastics.