Kinetics of aquation and base hydrolysis ofcis-(carboxylato) (ethylamine)bis(ethylenediamine)cobalt(III) ions

Abstract

The aquation ofcis-[Co(en)₂(NH₂Et)O₂CR]²⁺ [R=H or Me] is strongly acid–catalysed and the rate and activation parameters for this process are reported. No significant rate difference is observed in the spontaneous aquation path for the complexes. The acetato complex undergoes acid catalysed aquation at a rate comparable to the of the corresponding formato complex, in contrast with the relative basicities of the coordinated formate and acetate. This result is interpreted in terms of relative solvation effects of the initial and transition states of both complexes. The base hydrolysis of both complexes obeys overall second order kinetics in the 0.05 ≤ [OH^-]_T ≤ 0.35 mol dm^-3 range (I=0.5 mol dm^-3). The formato complex reacts five times faster than its acetato analogue under comparable conditions, which is fully consistent with the dissociative mode of activation of the amido conjugate base involving Co-O bond heterolysis. A substantially large positive value for the activation entropy supports SN₁CB mechanism for base hydrolysis.