Kinetics and mechanism of acetate and succinate anations ofcis-tetraamminediaquacobalt(III) in aqueous acidic media

Abstract

The complexcis-tetraamminediaquacobalt(III) reacts with acetate ion to formcis-tetraamminediacetatocobalt(III). Kinetic evidence shows that the reaction proceeds in a single step process in the pH range 4.0–5.2, but it splits to a biphasic one at pH 5.4. The acetate anation is governed by the rate law: $${\text{k}}_{{\text{obs}}} = {\text{k}}_{\text{0}} + \frac{{{\text{kQ[OAc}}^ - {\text{]}}}}{{1 + {\text{Q[OAc}}^ - {\text{]}}}}$$ where kobs, k0, k and Q denote the observed rate constant, the back aquation rate, the interchange rate and the ion-pair formation constant, respectively. The ΔH≠] and ΔS≠ values associated with the interchange rate are 137 ± 8 kJ mol−1 and 137 ± 12 JK−1mol−1, respectively. The title complex reacts with succinate ions (HSuc−1 and Suc2−) to form tetraamminesuccinatocobalt(III) and no evidence has been obtained for any significant build-up of the monodenate succinato intermediate. It is concluded that the step in which the succinate ion chelates is rapid. The kinetics of the first stage of the reaction involving coordination of succinate ion to cw-[Co(N-H3)4(OH2)2]3+ have been investigated at pH=2.5–4.0, I=0.5 mol dm−3 (NaClO4) and at temperatures 30–50 °C. The interpretation given is consistent with the formation of ion-pairs between the diaqua complex and the succinate ions, HSuc− and Sue2−. The reaction is believed to proceed through a dissociative interchange mechanism.