Kinetics and mechanism of the reductions of tris(oxalato)cobaitate(III) ion by ruthenium(II) species in aqueous solution

Abstract

The kinetics and mechanism of the reductions of [Co(C2O4)3]3– by the ruthenium(II) species [Ru(NH3)6]2+, [Ru(en)3]2+(en = ethylenediamine), and [Ru(NH3)5(H2O)]2+ in aqueous solution have been investigated at I= 0.20 mol dm–3(LiCl). The reductions are acid-independent in the range [H+]= 0.01–0.10 mol dm–3, and the second-order rate constants for the reductions at 25 °C are (17.8 ± 1.6)× 10–2, (5.5 ± 0.3)× 10–2, and (3.3 ± 0.3)× 10–2 dm3 mol–1 S–1 respectively. The activation parameters for the reductions by [Ru(NH3)6]2+ and [Ru(NH3)5(H2O)]2+ are ΔH‡= 45.4 ± 6.9 kJ mol–1, ΔS‡=–(107.6 ± 22.9) J K–1 mol–1 and ΔH‡= 98.4 ± 7.6 kJ mol–1, ΔS‡=+(56.5 ± 4.2) J K–1 mol–1 respectively. The reductions by [Ru(NH3)6]2+ and [Ru(en)3]2+ occur by an outer-sphere mechanism, while that by [Ru(NH3)5(H2O)]2+ occurs by a substitution-controlled inner-sphere mechanism.