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

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 compositions for a particular complex are linearly correlated. A common mechanism appears plausible for all the complexes in which bond breaking and bond making are synchronous but their relative contributions are different giving a relatively more associative character to the cobalt(III) and rhodium(III) systems compared to the chromium(III) system. This view is supported by the magnitudes of the slopes of the log k vs 1/D, and log k vs Y plots. The dissociation of the M-NCS bond is probably solvent assisted due to solvation of the departing ligand as indicated by the linear dependence of ΔH^≠ and ΔS^≠ on different solvent parameters.