BASE HYDROLYSIS OF ISO-THIOCYANATOPENTAAMMINE COMPLEXES OF COBALT(III), CHROMIUM(III) AND RHODIUM(III) IN AQUEOUS-ORGANIC MIXED SOLVENTS

Abstract

Abstract In the base hydrolysis of the title complexes in aqueous-organic solvent media (water-ethanol and water-acetone) to the corresponding hydroxo-complexes the observed rate law is in accord with the expression: The observed K values (3.5 to 26 M−1 at 25°) and the nature of their solvent dependence suggest the formation of an ion-pair in a pre-equilibrium involving the substrate complex and OH− ion, and not of a conjugate base. Rate constants (k) for the transformation of the ion-pair to the product increase with increased proportions of the organic component in the solvent. The nature of the dependence of k on different solvent parameters indicates significant associative character with OH− ion acting as the attacking nucleophile. Activation enthalpy, ΔH≠, decreases with ethanol concentration; this along with the observed large positive slopes of the log k vs 1/D plots rule out the possibility of H2O acting as the attacking nucleophile. ΔH≠ and ΔS≠ corresponding to k in different solvent composition...