Effect of solvent on the reactions of coordination complexes. Part 16. Kinetics and mechanism of complexation of oxalatopentaammine-cobalt(III) with nickel(II) in methanol-water medium

Abstract

The kinetics of reversible complexation of oxalatopentaammine cobalt(III) with Ni²⁺ has been investigated in MeOH + water media (0-50 (v/v) % MeOH) at 15.0-35.0°C and I = 0.10 mol dm^-3. Analysis of rate data indicates that the monobonded complex [(NH₃)₅ CoOCOCO₂Ni]³⁺ in which Ni²⁺is bound to the end carboxylate group is the possible reaction intermediate. The formation and dissociation rates of such a species are rate limiting for the overall formation and dissociation of the binuclear species, in which Ni²⁺ is chelated by the oxalate moiety. The rate and activation parameters for formation and dissociation of the binuclear species are moderately solvent sensitive and solvent structural effects are discernible in the nonlinear variations of ΔH^≠ and ΔS^≠ with solvent composition. The log k_r vs. Grunwald Winstein parameter (YBu^±cl) plot for the dissociation of the binuclear species is markedly nonlinear.