Effect of solvent on the reactions of coordination complexes. Part 9. Kinetics of solvolysis of cis-(chloro)(cyclohexylamine) bis(ethylenediamine)cobalt(III) in methanol -water, propan-2-ol-water, ethylene glycol-water and t-butyl alcohol-water media

Abstract

The kinetics of the solvolytic aquation of cis-(chloro)(cyclohexylamine)-bis(ethylenediamine)cobalt(III) ion were investigated in water-methanol, water-propan-2-ol, water -ethylene-glycol (0-80 wt % alcohol) and water-t-butyl alcohol (0-50 wt % alcohol) media at 35 -60 °C. Plots of log k^s_obsvs. D⁻¹s(where D_s is the bulk dielectric constant of the solvent mixture), log k^s_obsvs. the Grunwald -Winstein solvent parameter, Y_But_Cl, and log k^s_obsvs. X_org(= mole fraction of cosolvent) tended to be curved. Under isodielectric conditions (D_s≈ 50) at 50 °C, the plot of log k^s_obsvs. Xorg for methanol -water, ethanol -water, propan-2-ol-water, t-butyl alcohol-water, acetone-water, ethylene glycol -water also did not yield a satisfactory straight line. The observed dependence of solvolysis rate on X_org has been interpreted in terms of preferential solvation of the initial and transition states which is believed to be at least partly controlled by the overall acidity and basicity of the mixed solvent media as well as the hydrophobic interaction effect. Variation of enthalpies and entropies of activation with solvent composition was correlated with changes in the physical properties of the solvent mixtures. The solvent isotope effect (k_H2O/kD₂O= 1.38 ± 0.16 at 50 °C) was, however, normal as observed for several other cis-[Co(en)₂(primary amine)Cl]²⁺ complex ions.