Effect of solvent on the reactions of coordination complexes. Part 1.—Kinetics of solvolysis of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water media

Abstract

The kinetics of the solvolytic aquation of cis-(bromo)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated at 35, 40, 45, 50.2 and 54.8 °C in aqueous methanol media of methanol content 0–80 wt %. The pseudo-first-order rate constant decreases with increasing methanol content. Plots of log k^s_aqvs. D^–1_s(where D_s is the dielectric constant of the solvent mixture) and log k^s_aqvs. the Grunwald–Winstein Y parameter or revised Y_Cl(1-adamantyl Chloride) parameter are non-linear. The plot of log k^s_aqvs. the molefraction of methanol (X_MeOH), which deviates from linearity at 35, 40 and 45 °C, is an excellent straight line (r= 0.998) at 50 and 54.8 °C over the entire composition range studied. It is evident that the solvation phenomenon plays dominant role and that the rate of aquation is mediated by the dual solvent vectors, the overall basicity and acidity of the solvent mixtures. At 25 °C the calculated values of the transfer free energy of the dissociative transition state, cis-{[Co(en)₂(bzmH)]³⁺}^*, relative to that of the initial state, cis-[Co(en)₂(bzmH)Br]²⁺, for the transfer of the ions from water to the mixed solvent were positive at all solvent compositions (except 80 % MeOH), exhibiting a maximum at X_MeOH= 0.46. This reflects the propensity of the said tripositive transition state and the dipositive initial state towards solvation interaction with the medium. In contrast to the activation free energy, which varies linearly with X_MeOH, the plots of activation enthalpy and entropy against X_MeOH exhibit maxima at X_MeOH= 0.12 and 0.46 and minima at X_MeOH= 0.02 and 0.25; thereafter both ΔH^‡ and ΔS^‡ decrease steeply to significantly low values at X_MeOH= 0.69 (ΔH^‡= 80.1 ± 4.3 kJ mol^–1 and ΔS^‡=–91 ± 13 J K^–1 mol^–1), manifesting the effect of solvent-shell reorganisation around the complex ion both in the initial state and in the transition state as the composition is varied.