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Oxidation Number

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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