Effect of solvent on the reactions of coordination complexes part VII: Kinetics of solvolysis of cis-(bromo) (imidazole) bis-(ethylenediamine)cobalt (III) and cis-(bromo) (N-methyl imidazole)bis-(ethylenediamine) cobalt(III) in methanol-water media

The kinetics of the solvolytic aquation of cis-(Bromo) (imidazole) bis(ethylenediamine) cobalt (III) and cis-(Bromo) (N-methylimidazole) bis(ethylenediamine) cobalt(III) have been investigated in aqueous methanol media with methanol content 0–80% by weight and at temperatures 40–55°C. The pseudo-first order rate constant decreases with increasing methanol content. Plots of log k vs. D (where D_s is the bulk-dielectric constant of the solvent mixture) and log k vs. the Grunwald-Winstein Y-solvent parameter are nonlinear, the curvature of the plots is relatively more significant for the imidazole complex. The plots of log k vs. molfraction of methanol (X_MeOH) for both the substrates also deviate from linearity, the deviation being less and less marked, particularly for the N-methyl imidazole complex, as the temperature is increased. Hence preferential solvation phenomenon appears to be less significant when the N-H proton of imidazole is replaced by -CH₃ group. The plots of calculated values of the transfer free energy of the dissociative transition state, cis-{[(en)₂Co(B)]³⁺}* (B = imidazole, N-methylimidazole), relative to that of the initial state, cis-[Co(en)₂(B)Br]²⁺, for the transfer of the ions from water to the mixed solvent, against X_MeOH exhibit maxima at X_MeOH = 0.06, 0.27, and 0.12, 0.36 and minima at X_MeOH = 0.12 and 0.19 for cis-[(en)₂Co(imH)Br]²⁺ and its N-methylimidazole analogue respectively which are in keeping with the solvent structural changes around the initial state and transition state of these substrates as the solvent composition is varied. Plots of activation enthalpy and entropy against molfraction of the solvent mixtures exhibit maxima and minima. This type of variations of the activation parameters, ΔH^≠ and ΔS^≠, with X_MeOH speaks of the enthalpy and entropy changes associated with the solvent-shell reorganization of the complex ions both in the initial and in the transition states which contribute appreciably to the overall activation enthalpy and entropy of the aquation reaction.