Complex Compounds

COMPLEX COMPOUNDS belong to the numerous class of chemical substances, which by their empirical composition represent compounds of several simplest molecules, as for example crystal hydrates, ammoniates, double salts, etc., and whose structure cannot be explained with the help of our usual concepts of valence. All these compounds bear the common name "molecular" compounds (or compounds of "higher order") in contrast to the simplest compounds of atoms, formed with the participation of valence forces. Chemistry is indebted to the German scientist Werner (A. Werner) for elucidating the structure of C. s., who created the so-called coordination theory (see), which by means of certain positions made it possible to unite all molecular compounds into a coherent and clear system.

Depending on the strength of molecular compounds, the properties of the original molecules either change very slightly, as is observed for example in double salts, which almost completely dissociate in aqueous solutions into their components, or are completely lost, with their ability to ionize changing especially sharply. Thus for example, the very weakly dissociated molecule H₂O, forming C. s. with ammonia, gives OH- ions of ammonium hydrate (NH₄OH), while in the formation of chloroplatinates from PtCl₄ and chlorides, the characteristic properties of chloride ions disappear (for example in K₂PtCl₆). For especially strong C. s., it must be assumed that atoms or radicals located in the immediate vicinity of the central atom, in the first "zone" (see Coordination theory), are incapable of ionization, do not give their characteristic qualitative reactions, and together with the central atom form a complex "complex" ion, the charge of which is determined by the algebraic sum of the initial charges of the central nucleus and the atoms or acid residues coordinated around it, for example Cl-⁺ - ⁺ = [PtCl₆]^2-. Cl- + Cl-Cl-

If in the complex all coordination places are occupied only by complete neutral molecules, its charge is naturally equal to the charge of the central atom, for example [Ni²⁺(NH₃)]₆]²⁺; [Co³⁺(NH₃)₆]³⁺. Atoms and acid residues in the second sphere, in contrast to atoms and radicals of the first sphere, represent free ions, easily exchangeable for other ions of the same sign.

As stated above, atoms and radicals enclosed in the complex practically do not ionize; however, substitution reactions are also possible for them. These reactions, by their slight speed, very much resemble reactions of organic compounds. In this case, any of the groups entering into the complex can be replaced by either a single atom or a whole molecule or molecular residue. In such transformations, quite definite regularities can be observed. By successively replacing neutral molecules in the complex with acid residues or vice versa, one can pass from a complex ion of one sign to a complex ion with the opposite sign and obtain as a result a series of complex compounds, in the middle of which there should be a neutral compound that does not form ions. Such series of transformations are known for many complex compounds, for example [Co(NH₃)₆]Cl₃, [Co(NH₃)₅NO₂]Cl₃, [Co(NH₃)₄(NO₂)₂]Cl₃, [Co(NH₃)₃(NO₂)₃], [Co(NH₃)₂(NO₂)₄]K, [Co(NH₃)(NO₂)₅]K₂, [Co(NO₂)₆]K₃.

Hexammine-cobalt chloride, Nitro-pentammine-cobalt chloride, Dinitro-tetrammine-cobalt trinitro-triammine-cobalt chloride, Tetranitro-diammine-cobalt potassium (not obtained), Hexanitro-cobaltate potassium.

The gradual change in dissociation of these compounds can be measured by determining the electrical conductivity of their dilute solutions. The value of the molecular conductivity of the first compound corresponds to the conductivity of compounds dissociating into 4 ions, the second into 3 ions, the third into 2 ions, and the fourth turns out to be a non-electrolyte.

By gradually replacing the ammonia molecules in hexammine-chromium chloride [Cr(NH₃)₆]Cl₃ with water molecules, Werner established the connection between typical complex compounds and crystal hydrates, proving that the latter can also be classified as complex compounds as compounds in which all coordination places are replaced by water molecules.

[Cr(NH₃)₆]Cl₃, [Cr(NH₃)₅H₂O]Cl₃, [Cr(NH₃)₄(H₂O)₂]Cl₃, [Cr(NH₃)₃(H₂O)₃]Cl₃, [Cr(NH₃)(H₂O)₅]Cl₃, [Cr(H₂O)₆]Cl₃.

Hexammine-chromium chloride, Aquopentammine-chromium chloride, Diaquotetrammine-chromium chloride, Triaquotriammine-chromium chloride, Tetrakodiammine-chromium chloride, Pentakvoammine-chromium chloride, Hexakvo-chromium chloride.

To the class of complex compounds, Werner also attributed certain basic salts, formed by combining molecules of normal salt with several molecules of hydroxide, with the place of the central atom occupied by the metal of the normal salt, while the coordination places are occupied by hydroxide molecules. Thus for example for the salt CuCl₂·3Cu(OH)₂ (mineral atacamite) Werner gives the following coordination formula: [Cu²⁺(OH)₄]^2-, with each hydroxide molecule occupying two coordination places (coordination number of copper in this case = 6). For the basic calcium chloride of composition CaCl₂·3Ca(OH)₂·12H₂O the formula [Ca²⁺(OH)₄]^2- Ca(OH₂)₄ Cl₂] is obtained.

An extremely numerous group of compounds is formed by complexes, the composition of which includes molecules of organic substances. From biologically important compounds, one can mention heme, chlorophyll. Besides compounds with one central nucleus, complexes with several nuclei are known, the so-called "multinuclear complexes", an example of which are the basic salts already mentioned above. In such multinuclear complexes, the connection between the nuclei is carried out by means of hydroxyl (here called ol-groups), amide, imide, and other groups, with one of these groups being valently bound to one of the nuclei, for example [(NH₃)₄Co-O-Co(NH₃)₄]Cl₄ Octammine-diol-cobalt chloride.

Of the same kind are multinuclear complexes, which are poly- and heteropoly-acids, for example polychromates: [O₂Cr(CrO₄)₂]K₂, [OCr(CrO₄)₃]K₂, phosphotungstic and phosphomolybdic acids: [P(W₂O₇)₆]H₇, [P(Mo₂O₇)₆]H₇.

The complexity of the structure of complex compounds leads to the appearance of various cases of isomerism. We will dwell here on the following most studied types. 1. Coordination isomerism is observed in compounds with two (or several) complex ions (cation and anion) and is caused by the different distribution of groups coordinated around the central atom. For example for the compound with empirical composition CoCr(CN)₆·6NH₃ 2 isomers are obtained: 1) [Co(CN)₆][Cr(NH₃)₆], 2) [Co(NH₃)₆][Cr(CN)₆]. 2. Hydrate isomerism occurs in some crystal hydrates and is caused by the different position of water molecules. A vivid example of such isomerism can be the isomeric hydrates of chromium chloride-CrCl₃·6H₂O. One of them-violet in color-corresponds to the formula [Cr(H₂O)₆]Cl₃, two others-green in color-to the formulas: [Cr(H₂O)₅Cl]Cl₂·H₂O and [Cr(H₂O)₄Cl₂]Cl·2H₂O. 3. Ionization isomerism is characteristic of C. s., the composition of which includes several different acid residues. They can be either in the first or in the second sphere, due to which a substance of the same composition in aqueous solutions dissociates into different ions, for example [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br. 4. Salt isomerism is determined by the isomeric forms of the acid residues entering into the complex, for example the residue of nitrous acid, which can enter both as nitro- (NO₂) and as nitrito- (ONO) groups. 5. Of all cases of isomerism, however, spatial isomerism, or stereoisomerism, is of the greatest interest, brilliantly confirming Werner's idea of the spatial arrangement of groups enclosed in the complex (about this-see Coordination theory).