The largest and most diverse among inorganic substances is the class of complex compounds. A group of organometallic substances, such as chlorophyll and hemoglobin, can also be attributed to it. It is these compounds that are the bridge that combines 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 is invaluable, in the study of the most important biological processes: photosynthesis, internal (cellular) respiration.
In this article, we will study the structure and nomenclature of complex compounds, as well as consider the basic principles of their classification.
Coordination theory of A. Werner
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, which were respectively called the central ion, ligands (addends) and the external coordination sphere. In order to make the classification and nomenclature of complex compounds clear, we will analyze these concepts in more detail. So, A. Werner proved the presence in the molecule of an ion (usually positively charged), occupying a central position. It began to be called a complexing agent, a central ion or 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 remaining particles that are not included in it form the outer shell of the molecule.
So, in the formula of sodium cuprite Na 2 [Cu (OH) 4 ], the central copper atom in the oxidation state is +2 and four hydroxo groups make up the inner sphere, and sodium ions are located at some distance from the central atom in the outer sphere.
Methods for determining coordination formulas and substance names
To date, the theory of A. Werner remains the main theoretical basis on the basis of which complex complex compounds are studied. The nomenclature, that is, the names of these substances, are determined by the rules adopted by the International Society of Theoretical and Applied Chemistry.
Here are a few examples of formulas of substances in which the complexing agent is represented by a platinum atom - K 2 [PtCl 6 ] or NH 3 molecules - [Ag (NH 3 ) 2 ] Cl. As it turned out, the formulas can be derived using the following practical methods: double exchange reactions, the molar conductivity of solutions, and X-ray diffraction. Consider these methods in more detail.
As proved the structure of complex compounds of platinum
The substances of this group are characterized by the presence of platinum in the central atom of the molecule. If the PtCl 4 × 6NH 3 compound is acted upon by a solution of silver nitrate, then all the chlorine present in the substance binds to the metal atoms and white flakes AgCl are formed. This means that all chlorine anions were in the external coordination sphere, while ammonia molecules were bound to the central atom of platinum and together with it formed the inner sphere.
So, the coordination formula of the substance will be written in the following form: [Pt (NH 3 ) 6 ] Cl 4 and called hexammin platinum 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 chromium compounds
The structure of substances of this group was determined using the physical process of x-ray diffraction, which underlies the X-ray diffraction analysis. Passing through the crystal lattice, electromagnetic waves are scattered by the action of electrons of the substance under study. This makes it possible to very accurately establish which groups of atoms are located in the nodes of the crystal lattice. For chromium-containing crystals, an appropriate nomenclature of complex compounds was created. Examples of the names of the isomeric hydrates of trivalent chromium salts compiled using the X-ray diffraction method are as follows: tetraacvadichlorochromium (III) chloride, pentaaquachlorochromium (III) chloride.
It was found that in these substances the chromium atom is associated with six different addenda. How is this indicator determined and what factor affects the coordination number?
How the central atom is bonded to 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 receipt, classification and nomenclature of complex compounds directly depend on this indicator. It is correlated with the degree of oxidation of the central atom. In compounds of platinum, chromium, and iron, the coordination number is most often six; if the complexing agent is represented by copper or zinc atoms, four; if the central atom is silver or copper, two.
Types of complex compounds
In chemistry, one distinguishes between the main classes and transitional series of substances between them. Complex compounds considered in previous subheadings, the nomenclature of which indicates the presence of water molecules in their structure, belong to aquacomplexes. Ammonia include substances containing neutral particles of ammonia, for example, triiodine triamminrodium. The class of chelating compounds is peculiar in the structure of molecules. Their name comes from the biological term chelicera - the so-called claws of decapod crayfish. These substances contain addends, the spatial configuration of which covers the complexing agent, like claws. Such compounds include the ferric oxalate complex, the ethylene diamine complex of platinum with an oxidation state of +4, and the salts of aminoacetic acid, which include rhodium, platinum or copper ions.
Rules for the compilation of the names of complex compounds
The most common control question in chemistry assignments in a high school course is: name complex compounds according to the IUPAC nomenclature. For a specific example, we analyze the algorithm for compiling the name of a substance having the following formula: (NH 4 ) 2 [Pt (OH) 2 Cl 4 ].
- The name begins with determining the composition of the internal coordination sphere. It contains anions of hydroxyl groups and chlorine. We add the ending –o to their names. We get: dihydroxo-, tetrachloro-.
- Now we will find the complexing agent, using the Latin name for its designation, and add the suffix –at to it, in parentheses we indicate its oxidation state: platinum (IV).
- Having finished with the designation of the inner sphere, we move on to the outer part. We will call it cations: in our example, these will be ammonium ions.
As a result, the substance will have a name in which all of the above structures are indicated.
The use of complex compounds
At the beginning of the article, we called the most important representatives of organometallic substances, such as hemoglobin, chlorophyll, and vitamins. They play a leading role in metabolism. Complex compounds are widely used in technological cycles of smelting ferrous and non-ferrous metals. An important role in metallurgy is played by carbonyls, special complex compounds whose nomenclature indicates the presence of carbon monoxide CO in their molecules as an add-on. When heated, these compounds decompose and reduce metals such as nickel, iron, cobalt from their ores. Most complex compounds are also used as catalysts in the reactions of varnishes, paints and plastics.