Effect of solvent on the reactions of coordination complexes. Part 5. Kinetics of solvolysis of cis-(bromo)-[(2-aminothiazole)-bis(ethylenediamine)cobalt(III) in methanol-water, propan-2-ol-water and ethylene glycol-water

Abstract

The rates of solvolysis of the cis-(bromo)(2-aminothiazole)(ethylene-diamine)cobalt(III) ion have been investigated in acidic methanol-water, propan-2-ol-water and ethylene glycol-water (0-80 wt% of organic cosolvent) at 30-50 °C. A good linear correlation was observed between log k^s_obs and the Grunwald-Winstein Y parameter for MeOH-water and propan-2-ol-water. The log k^s_obsvs. 1/D_s(D_s= bulk dielectric constant of the mixed solvent) plots at 25 °C exhibited distinct curvature; the effect is pronounced for propan-2-ol-water. log k^s_obsvs. mole fraction of organic solvent component (X_org) plots for MeOH-water were linear at 30-50 C and X_MeOH= 0-0.692, while similar plots for ethylene glycol-water and propan-2-ol-water were biphasic (two intersecting lines); the effect is relatively more pronounced for ethylene glycol-water. The dependence of the solvolysis rate on Xorg has been interpreted in terms of solvation of the initial state and the transition state, which is controlled by the overall acidity and basicity of the mixed solvent. Activation enthalpy and entropy vs.X_org plots display maxima and minima, indicating that the solvent structural changes play a significant role in the activation process. The solvolysis reaction is isoenthalpic and more or less isentropic at X_org= 0.225 for all three aqua-organic solvent mixtures. The observed mutual compensation effect of activation free energy shows that the perturbations in the solvent network causes proportionate perturbations in the reaction zone so that the solvational components of ΔH≠ and ΔS≠ correlate with each other linearly.