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

Abstract

The kinetics of reversible complexation of Ni(OH2)62+ with oxygen-bonded glycinatocobalt(III) substrates N4Co(glyH)gly2+ [N4 = (en)2 or trien; glyH = H3N+CH2COO-)] have been investigated by the stopped-flow technique 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 triethylenetetraminecobalt(III) substrates, respectively. This result indicates that the rate and activation parameters are virtually independent of the nature of N4 moities, which strongly suggests that the formation of mono-bonded species occurs via entry of one of the pendant NH2 groups into the coordination sphere of nickel(II) via a rate-limiting Ni-OH2 bond dissociation mechanism (I(d)). The binuclear species exist in dynamic equilibrium between the monodentate and chelated forms, with the chelate form predominating. The low values of spontaneous dissociation rate 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 literature also support the chelate nature of the binuclear species.