KINETICS AND MECHANISM OF COMPLEX-FORMATION BETWEEN O-BONDED(GLYCINATO)PENTAAMMINECOBALT(III) AND (GLYCINATO)TETRAETHYLENEPENTAAMINECOBALT(III) IONS WITH NICKEL(II) IN AQUEOUS-MEDIUM

The kinetics of the reversible complexation of Ni(OH2)62+ with oxygen bonded glycinatocobalt(III) substrates, N5Co(glyH)3+ (N5 = 5 NH3 or tetraethylenepentamine; glyH = H2NCH2COOH) have been investigated by stopped flow technique at 25-degrees-C, I=0.3 mol dm-3. The values of formation and dissociation rate constants (k(f), k(r)) for the binuclear species, [N5CoO2CCH2NH2Ni] are 270 +/- 37, 271 +/- 41 dm3 mol-1 s-1, and 0.140 +/- 0.015, 0.100 +/- 0.007 s-1 at 25-degrees-C (I=0.3 mol dm-3) for te tetraethylene-pentaminecobalt(III) and pentaamminecobalt(III) substrates respectively. Lack of any dependence of k(f) on the nature of the cobalt(III) moiety strongly suggests that the formation of the mono bonded species occurs by the entry of the pendant NH2 group into the coordination sphere of Ni(II) via rate limiting Ni-OH2 bond dissociation mechanism (I(d)). The binuclear species exist in dynamic equilibrium between the monodentate and chelated forms with chelate form predominating. The small values of the dissociation rate constants despite the intrinsic electrostatic repulsion between the like charge centres also support the chelate nature of the binuclear species.