Kinetics and mechanism of the reduction of penta-amminehalogenocobalt(III) complexes by ruthenium(II) species in aqueous solution

Abstract

The reactions of the penta-amminehalogenocobalt(III) complexes, [CO(NH₃)₅X]²⁺(X = F, Cl, Br, or I), with the ruthenium(II) species, [Ru(en)₃]²⁺(en = ethylenediamine), [Ru(NH₃)₆]²⁺, and [Ru(NH₃)₅(H₂O)]²⁺ have been investigated in aqueous solution at an ionic strength 0.20 mol dm^–3[Li(ts); Hts = toluene-p-sulphonic acid]. The reactions are acid-independent in the range [H⁺]= 0.01–0.20 mol dm^–3. Both [Ru(en)₃]²⁺ and [Ru(NH₃)₆]²⁺ react with all the penta-amminehalogenocobalt(III) complexes by an outer-sphere mechanism. The second-order rate constants (k_F= 3.3 × 10^–2 dm³ mol^–1 s^–1, k_Cl= 18.0 × 10^–2 dm³ mol^–1 s^–1) for the reduction of the fluoro- and chloro-penta-amminecobalt(III) complexes by [Ru(NH₃)₅(H₂O)]²⁺ lie in the characteristic substitution rate range for [Ru(NH₃)₅(H₂O)]²⁺ and therefore suggest an inner-sphere mechanism for these reactions. Moreover, the reactions involving the fluoro- and chloro-complexes deviate from log-log plots of reactions involving some Co^III complexes and ClO₄^–. This is the first instance that [Ru(NH₃)₅(H₂O)]²⁺ is shown to behave as an inner-sphere reductant. The bromo- and iodo-penta-amminecobalt(III) complexes react with [Ru(NH₃)₅(H₂O)]²⁺ by an outer-sphere mechanism, however. Activation parameters measured for the reactions of [Ru(en)₃]²⁺ and [Ru(NH₃)₆]²⁺ with the penta-amminefluorocobalt(III) complex are: ΔH^‡=(29.4 ± 2.3) kJ mol^–1, ΔS^‡=–(1 20 ± 15) J K^–1 mol^–1 and ΔH^‡=(47.1 ± 4.2) kJ mol^–1, ΔS^‡=–(50 1 ±. 6 0) J K^–1 mol ^–1 respectively.