In classical inorganic chemistry it is assumed that each metallic element can form a positive ion (cation) when one or more of the outer electrons is removed or transferred to another element. The number of these electrons removed or added is also equal to the difference between the total number of electrons in the neutral atom and the number associated with the same element in the ion form. This value is called the oxidation state, a number which describes the combining power of the atom and was in the past called "valence." Thus lead with an oxidation state of +2 will combine with two univalent negative ions (anions) or one divalent anion to form compounds like lead chloride or lead sulfate: PbCl2 or PbSO4. These molcules are held together by electrostatic forces, and, of necessity, are electrically neutral, the sum of the positive charges of the cations equalling the sum of the negative charges of the anions. In aqueous solutions these neutral molecules can dissociate into electrically conducting ions, each carrying a charge which is an indication of its oxidation state.

The transition element ions are characterized by their variable oxidation states and their ability to form complex ions. Examples of the former property include iron as ferrous (+2) and ferric (+3). Manganese may be found as manganous with an oxidation state of +2, the unstable manganic, an oxidation state of +3, or even with an oxidation state of +7 in permanganate. In this group cations with incomplete orbitals which can accept at least two pairs of unshared electrons easily form complex ions. In the laboratory these may be made from a dilute solution of a transition element ion by the addition of a large excess of a concentrated solution of ammonia, or of an anion of either fluoride, chloride, bromide, iodide, cyanide or thiocyanate. Here the combining ratio is greater than would be expected by simple stoichiometric calculations using the common oxidation states of the two ions. The term coordination number, usually twice the highest oxidation state of the cation, gives an indication of the number of molecules or univalent anions that are held around the central cation in these complex ions. Once formed, the complex is held together by coordinate covalent bonds and thus acts as a unit. Perhaps the best known example is the addition of ammonia to a solution of pale blue copper sulfate. The resulting deep blue solution is the cupric ammonia complex ion (I).

Cu++ + 4 NH2 = Cu (NH3)4 2+ (I)

Cadmium with an oxidation state of +2 will combine with four molar quantities of fluoride ion to form the cadmium fluoride complex (II), and cobaltic (+3) will complex with six molar quantities of chloride ion (III).

Cd2+ + 4 F- = Cd F42- (II) Co3+ + 6 CI- = CoCl63- (III)

The final net charge on the complex ion will be the algebraic sum of the total positive charge of the cation and the negative charges of the anions; copper with an oxidation state of +2 combines with four neutral ammonia molecules forming a complex ion which has a net charge of +2, whereas cadmium with an oxidation state of + 2 combines with four fluoride ions, each of charge -1, forming the complex ion with net charge of -2. Also dissolved in the solution will be sufficient oppositely charged un combined ions of adequate number to make the final solution neutral. Usually the copper complex ion is represented as either Cu(NH3)42+(I) or in expanded form (IA):

Anions in a complex salt need not all be the same, and in some complex salts, using the cobalt amine complex as examples, there may be various combinations of groups, i.e.,

Co [ (NH3)6Cl]++ (IV), Co [ (NH3)4Cl2]+ (V), Co [ (NH3)2 (NO2)4]-1 (VI)

Each of these Wemer salts (named after Alfred Werner who recognized them in 1893) will have a different oxidation state for cobalt, but the same coordination number.

The phenomenon of coordination number can be somewhat explained by modern theories. The bonds between atoms can be associated with an overlap of atomic orbitals and from the electron distribution around each cation a definite geometry is obtained. The value of this coordination number, which is variable, depends somewhat on the relative radii of the central and surrounding atoms. The most common numbers are 4 and 6 but the range is from 2 to 10. The geometric shapes are either square coplanar or tetrahedron for elements with an oxidation state of +2 and coordination number of 4; and for atoms with the common oxidation state of +3 and coordination number of 6 the eight faced figure, octahedral. Other figures may be possible for larger atoms (see Table 1). Isomerism is possible in these molecules, and both optical and cis trans isomers have been isolated.

Table 1. Coordination Number And Geometric Shape Of Some Ions

The copper ammonia complex can be represented by (I), (IA), or by the three dimensional drawing (IB); cadmium fluoride complex ion (II) and the ammonia chloride complex ion of cobalt (V) may be represented as (HA) and (VA).

If, instead of having a free ammonia group at each corner of the square in the copper complex, two ammonia groups were joined together by an ethyl group, the coordinating molecule would be ethylenediamine, but the arrangement would be the same as that of the copper ammonia complex. This new complex is composed of a metal ion and an organic moiety and is in the form of a ring (VII). Complexes wherein two or more groups of a single organic compound are coordinated with a central cation resulting in the formation of a ring are called chelates.