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, kobs, at low [H+] and [NCS–] is of the form (ii), with k2 small and negligible at high [H+] and [NCS–]. kobs=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.