Solvent effects on the anation ofcis-diaquo-bis-ethylenediamine cobalt(III) by L-proline

Abstract

The anation kinetics of the title complex were investigated spectrophotometrically in aqueous methanol, ethanol, ipropanol, t-butanol and dioxane, and the following rate law was established: $$d[Co(en)_2 (P)^{2 + } ]/dt = \frac{{k_a K_E [cis - Co(en)_2 (H_2 O)_2^{3 + } ]_T [P.H]}}{{1 + K_E [P.H]}}$$ An increase in proline concentration led to an increase in reaction rate with a limiting rate at higher ligand concentration due to ion-pair formation, where KE=the ion-pair equilibrium constant of the substrate proline complex. The nature of KE dependence on solvent composition is in good agreement with the ion-pair formation; an increase in the solvent organic component increasing KE. The anation rate constants (ka) are influenced by the solvent composition and decrease as the dielectric constant diminishes. In an isodielectric medium the anation rate remains constant. A good relationship between anation rate constant with Grunwald-Winstein coefficient (Y) is observed. Activation parameters (ΔH≠ and ΔS≠) have been calculated and compared with the isotopic water exchange process and a dissociative interchange (Id) mechanism involving ion-pair formation is proposed.