Octahedral cobalt(III) complexes of the chloropentammine type. Part VI. Kinetic evidence for the ion-pair mechanism of base hydrolysis of cobalt(III) complexes in aqueous solution

Abstract

The replacement, by hydroxide ion, of the co-ordinated chloride in the chloropentamminecobalt(III) cation in aqueous solution has been studied over a wide range of alkali concentrations. The kinetics were done with excess of hydroxide ion at a constant ionic strength so that pseudo-first-order rate constants were obtained in all the runs. The rate constants are non-linear in hydroxide concentration, and this kinetic dependence may be explained on the basis of the ion-pair mechanism, according to which the reaction between the cobalt(III) cation and the hydroxide ion proceeds by a pre-equilibrium formation of an “intimate” ion-pair between the two ions, followed by a rate-determining rearrangement within the ion-pair. The above investigations are extended to other similar complexes, where it is shown that, when four ammonia groups in the chloropentamminecobalt(III) cation are changed to two ethylenediamine ligands, and thereafter to one triethylenetetramine ligand, the nonlinearity in the dependence of observed first-order rate constants on hydroxide concentration becomes less and less noticeable. This observation constitutes evidence against the conjugate-base mechanism for the base hydrolysis of octahedral cobalt(III) complexes in aqueous solutions.