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 (kF= 3.3 × 10–2 dm3 mol–1 s–1, kCl= 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 CoIII 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.