KINETICS AND MECHANISM OF COMPLEX-FORMATION BETWEEN O-BONDED CIS-(DIGLYCINATO)BIS(ETHYLENEDIAMINE)COBALT(III) AND CIS-(DIGLYCINATO)(TRIETHYLENETETRAMINE)COBALT(III) WITH NICKEL(II) IN AQUEOUS-MEDIUM

The kinetics of reversible complexation of Ni(OH2)62+ with oxygen-bond ed glycinatocobalt(III) substrates N4Co(glyH)gly2+ [N4 = (en)2 or trie n; glyH = H3N+CH2COO-)] have been investigated by the stopped-flow tec hnique in the 20-35-degrees-C range, at pH = 6.08-6.82 and I = 0.3 mol dm-3. The formation of N4Co(glyH)glyNi4+ occurred via the reaction of Ni(OH2)62+ with the deprotonated form of the cobalt(III) substrates, N4Co(glyH)gly2+. The rate and activation parameters for the formation and dissociation of the binuclear species are reported. The formation rate constants k(f) (at 25-degrees-C), activation enthalpy and entropy DELTAH(not-equal), DELTAS(not-equal) for N4Co(glyH)glyNi4+ are 320 +/ - 49, 341 +/- 52 dm3 mol-1 s-1, 78 +/- 7, 79 +/- 5 kJ mol-1 and 64 +/- 24, 69 +/- 18 J K-1 mol-1, for the ethylenediamine and triethylenetet raminecobalt(III) substrates, respectively. This result indicates that the rate and activation parameters are virtually independent of the n ature of N4 moities, which strongly suggests that the formation of mon o-bonded species occurs via entry of one of the pendant NH2 groups int o the coordination sphere of nickel(II) via a rate-limiting Ni-OH2 bon d dissociation mechanism (I(d)). The binuclear species exist in dynami c equilibrium between the monodentate and chelated forms, with the che late form predominating. The low values of spontaneous dissociation ra te constant for the binuclear species (k(r) congruent-to 0.095 s-1 at 25-degrees-C) in comparison with the high values of dissociation rate constants of monodentate nickel(II) complexes reported in the literatu re also support the chelate nature of the binuclear species.