Reactions of µ-hydroxo-dicobalt(III) complexes. Part XIII. Further kinetic studies on the reaction of thiocyanate with the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex

Abstract

Further studies have been made on the reaction of the µ-amido-µ-hydroxo-bis[tetra-amminecobalt(III)] complex with a large excess of thiocyanate, (i), in aqueous perchloric acid solutions I= 2.0 M(NaClO₄). [graphic omitted] + H⁺+ 2NCS^–→ [graphic omitted] + H₂O (i) Hydroxo-bridge cleavage is observed, where the concentration dependence of pseudo-first-order rate constants, k_obs, at low [H⁺] and [NCS^–] is of the form (ii), with k₂ small and negligible at high [H⁺] and [NCS^–]. k_obs=k₁[H⁺][NCS^–]+k₂[NCS^–](ii) A non-linear dependence on hydrogen-ion concentration has to be taken into account with [H⁺] 0.5M. At 25 °C, k₁=(4.7 ± 0.1)× 10^–4 I² mol^–2 s^–1, and activation parameters are ΔH^‡₁= 17.5 ± 0.4 kcal mol^–1 and ΔS^‡₁=–14.7 ± 1.4 cal K^–1 mol^–1.