Kinetics and mechanism of the reduction of thiocyanato-, isothiocyanato-, and azido-penta-amminecobalt(III) by penta-ammineaquaruthenium(II) in aqueous solutions

Abstract

Kinetic measurements have been made on the reactions of [Ru(NH₃)₅(OH₂)]²⁺ with [Co(NH₃)₅X]²⁺(X = SCN, NCS, or N₃) in aqueous solution. The reactions obey second-order rate laws with the following kinetic parameters at 25 °C, I= 0.20 mol dm^–3(LiCl), and [H⁺]= 0.10 mol dm^–3(HCl): k_SCN=(1.7 ± 0.10)× 10², k_N3= 0.61 ± 0.02, and k_NCS= 0.28 ± 0.01 dm³ mol^–1 s^–1; ΔH^‡_SCN= 35.1 ± 2.9 kJ mol^–1, ΔS^‡_SCN=–84.5 ± 7.5 J K^–1 mol^–1, ΔH^‡_N3= 55.2 ± 1.7 kJ mol^–1, ΔS^‡_N3=–(63.2 ± 8.4) J K^–1 mol^–1, ΔH^‡_NCS= 61.9 ± 3.3 kJ mol^–1, ΔS^‡_NCS=–47.3 ± 5.9 J K^–1 mol^–1. The reactions are not affected by added acid or chloride ion. The small ratio k_N3 : k_NCS obtained for these reactions confirms that they occur by an outer-sphere mechanism. In agreement with other outer sphere reactions previously reported, the ratio k_SCN : k_N3 for these reactions is larger than that in inner-sphere redox reactions and an excellent linear free-energy correlation is obtained when the modified form of the Marcus equation is used to compare the rates of the reduction of the cobalt(III) complexes by [Ru(NH₃)₅-(OH₂)]²⁺ and [Ru(NH₃)₆]²⁺.