It is a number (numerical value which represents the electric charge on the central metal atom when it is attached to other ligands. For example, the oxidation number of cobalt in [Co(NH₃)₆]³⁺ is + 3 and that of nickel in Ni(CO)₄ is zero.

It has been observed by the authors that many students find it difficult to determine the oxidation state and coordination number (CN) of a metal atom or ion in a complex. We give here few solved examples as illustrations:

(a)    K4[Fe(CN)₆]. (i)  This complex is composed of two parts cation and anion. The four K⁺ ions (+4) constitute the cations. Therefore, the anion must carry four negative charge [Fe(CN)₆]^4- . Since there are six cyanide ions directly attached to iron atom, therefore its CN is 6.

(ii) The cyanide ion is a uni-negative ligand. Therefore, six cyanide ions will constitute -6 charge. However, the complex-ion carries only-4 charge on it. This suggest that the oxidation state of iron atom must be +2, i.e., [-4 = (-6) + (+2)].

(b)    [Cr(C₂O₄)₃]^3- . The complex-ion carries-3 charge. Since oxalate ligand is a dinegative ion (C₂O₄)^2-, therefore three oxalate ligands carry a total of -6 charge. However, the complex ion has only-3 charge. This suggests that the oxidation state of chromium atom must be+3, i.e., [-3 = (-6) + (+3)].
The oxalate ion is a bidentate ligand and since the complex-ion has three bidentate oxalate groups, therefore, CN of Cr is 6.

(c)    [Co(NH₃)₃(NO₂)₃]. It is a non-ionic (or molecular) complex and carries no charge (neutral)on it. The cobalt atom is attached to three neutral, NH₃ molecules and three uni-negative nitro grops (NO₂^-). Since the molecule on the whole is neutral, the -3 charge of three nitro groups must be balanced by the +3 charge on cobalt. Thus, the oxidation state of Co is +3 and its CN is 6.

(d)    [Pt(en)(H₂O)₂(NO₂)(CI)CI₂. The complex is composed of two parts : The complex ion [Pt(en)(H₂O)₂(NO₂)CI] is the cation and the two CI- ion constitute the anionic part. Since the anionic part carries -2 charge therefore the cation, i.e., complex-ion must carry +2 charge, [Pt(en)(H₂O)₂(NO)₂(CI)²⁺.

The complex-ion contains (i) ethylenediamine (en) which is neutral (ii) two aquo (water) molecules which are also neutral (iii) a nitrc group which carries-1 charge and (iv) a chloro group which also carries -1 charge. Although the ligands carry-2 charge but on the whole the complex-ion carries+2 charge. This suggests that Pt carries +4 cahrge, [(+4) + (-2) = -2]. The oxidation state of Pt is +4.

The complex-ion [Pt(en)(H₂O)₂(NO₂)CI]²⁺ contains one bidentate ligand (en) and four unidenate ligands. Thus the CN of Pt = 6.

(e)    Ni(CO)₄. The complex contains four carbonyl groups so its CN is 4.

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