The trans-[Co(meen)2C12]C1 complex was prepared and characterized by elemental analysis, and u.v. vis. and i.r. spectroscopies. The kinetics of the primary aquation of trans-[Co(meen)2C12] + in H20 and H20-MeOH have been investigated over a wide range of solvent compositions and temperatures (45-60 ~ Plots of rate constants (log k) versus the reciprocal of the dielectric constant of the medium (D~ -1) and Grunwald Winstein values of the solvent (Y) were non-linear. The variation of enthalpies (AH*) and entropies (AS*) of activation with solvent composition have been determined. Plots of AH* or AS* versus the mole fraction of the solvent exhibit a maximum at x2 ca. 0.1 and a minimum ofx 2 ca. 0.3; a linear plot of AH* versus AS* is obtained. Furthermore, the cycle relating the free energy of activation in H20 to that in H20-MeOH shows that changes in the solvent structure in H20-MeOH mixtures generally stabilize the five-coordinate cation in the transition state, more than the cation in the initial state as the mole fraction of MeOH increases. The results are discussed and compared with other related systems.

The aquation of K-[Co(dien)(en)Cl] 2+ was followed spectrophotometrically within the temperature range (40-60 • C) in water, water-isopropyl alcohol, and water-tert-butyl alcohol media of varying solvent composition up to 50 and 60 vol% of the organic solvent component respectively. The nonlinear plot of log k vs. D −1 s was attributed to the differential solvation of the initial and transition states. The variation of H = , S = , and G = with the mole fraction of the organic component was analyzed and discussed. The isokinetic temperatures were found to be 330 and 317 K for water-isopropyl alcohol and water-tert-butly alcohol mixtures respectively, indicating that the aquation reaction is entropy controlled. The application of free energy cycle at 25 • C for the aquation reaction in both co-solvents suggests that the transition state is more stable than the initial one.