Reactions of µ-hydroxo-dicobalt(III) complexes. Part VIII. Kinetic studies of the bridge cleavage of the µ-hydroxo-bis[penta-amminecobalt(III)] complex

Abstract

The kinetics of bridge cleavage of the µ-hydroxo-bis[penta-amminecobalt(III)] complex, (NH3)5Co·OH·Co(NH3)55+[equation (i)] have been studied in aqueous perchloric acid solutions, I= 2·0M(LiClO4), at 15–30 °C. (NH3)5Co·OH·Co(NH3)55++ H++ H2O → 2Co(NH3)5H2O3+(i) For the range of hydrogen-ion concentration investigated (10–4–2·0M), first-order rate constants, kobs, show the dependdence (ii) on [H+]. kobs=k1[H+]+k2(ii) At 25 ° C: k1=(5·37 ± 0·07)× 10–3 l mol–1 s–1, ΔH1‡= 12·0 ± 0·8 kcal mol–1, ΔS1‡=–28·7 ± 2·7 cal K–1 mol–1; and k2=(7·58 ± 0·10)× 10–3 s–1, ΔH2‡= 20·1 ± 0·70 kcal mol–1, ΔS2‡=–1·0 ± 2·2 cal K–1 mol–1. In the presence of thiocyanate ions (0·06–0·80M), Kobs, can be expressed as in (iii), where a and bare ca. 15% smaller than k1 and k2 due to ion-pairing. kobs=a[H+]+b+c[H+][NCS–](iii) At 25 °C and I= 2·0M(LiClO4) : c= 0·137 ± 0·003 I2 mol–2 s–1, ΔHc‡= 10·9 ± 0·6 kcal mol–1, and ΔSc‡=–25·8 ± 2·2 cal K–1 mol–1. The thiocyanate-dependent path yields equal amounts of the complexes Co(NH3)5NCS2+ and Co(NH3)5H2O3+ as primary products.