Decarboxylation kinetics for the carbonatotris(pyridine)cobalt(III) ion in perchloric acid solutions

Abstract

The kinetics of the decomposition reactions of the CO(py) 3(CO 3)(H 2O) + ion have been investigated in aqueous perchloric acid solutions over a range of hydrogen ion concentrations (0.10 to 5.0 M) and at two ionic strengths ( I = 1.0 and 5.0 M). At the lower ionic strength, plots of ln ( A t– A ∞ versus time show a nonlinearity that is consistent with that expected for consecutive first-order reactions. The rates of the faster reaction are similar to those reported for the spontaneous reduction of aquopyridine-cobalt(III) cations. At the higher ionic strength, the above noted curvature is not apparent and the decarboxylation kinetics of the title complex may be described by a pseudo-first-order rate constant: k obs = k[H 3O +]. At 20°C, k = (1.75−+0.09) s −1 M −1 with activation parameters ofΔ H ‡ = (97 −+ 4) kJ mol −1 and ΔS ‡ = −(54 −+ 32) J deg −1 mol −1. These kinetic parameters are compared with those previously reported for the similar complexes, Co(py) 4CO 3 + and Co(py) 2(CO 3)(H 2O) 2 +.