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.