Structural and mechanistic studies of co-ordination compounds. Part VI. Preparation, aquation, and base hydrolysis of some octahedral trans-chlorocyanocobalt(III) amine complexes

Abstract

The preparation of the complex trans-[Co(NH₃)₄(CN)Cl]⁺ is described. The kinetics of aquation and base hydrolysis of this complex and the base hydrolysis of the complex trans-[Co (cyclam)(CN)Cl]⁺(cyclam = 1,4,8,11-tetra-azacyclotetradecane) have been studied over a range of temperature, and the following parameters have been obtained: trans-[Co(NH₃)₄(CN)Cl]⁺, k_OH2(at 25 °C)= 9·8 × 10^–4 s^–1, ΔH_OH2^‡= 20·7 kcal mol^–1, ΔS_OH2^‡=–3 cal K^–1 mol^–1, and k_OH(at 0 °C)= 3·5 × 10^–2 l mol^–1 s^–1, ΔH_OH^‡= 23·8 kcal mol^–1, ΔS_OH^‡= 22 cal K^–1mol^–1; trans-[Co(cyclam)(CN)Cl]⁺,K_OH(at 0 °C)= 8·1 × 10^–1 l mol^–1 s^–1, ΔH_OH^‡= 22·6 kcal mol^–1, ΔS_OH^‡= 24 cal K^–1 mol^–1. Thermodynamically, it was found that the extent of aquation of complexes of the type trans-[Co(amine)₄(CN)Cl]⁺ increases in the order cyclam < (en)₂ < (NH₃)₄(en = ethylenediamine). Kinetically, the aquation rate constants of these complexes increase, but the base-hydrolysis rate constants decrease in the above order. The results support the proposed nephelauxetic effects which these amine ligands exert on the central cobalt(III) ion. Spectroscopic evidence is produced to support the above conclusion.