KINETICS OF AQUATION OF THE S-DICHLOROBIS(N-METHYLETHYLENE-DIAMINE)COBALT(III) ION IN A WATER-METHANOL MEDIA

The trans-[Co(meen)(2)Cl-2] Cl complex was prepared and characterized by elemental analysis, and u.v.-vis. and i.r. spectroscopies. The kine tics of the primary aquation of trans-[Co(meen)(2)Cl-2](+) in H2O and H2O-MeOH have been investigated over a wide range of solvent compositi ons and temperatures (45-60 degrees C). Plots of rate constants (log k ) versus the reciprocal of the dielectric constant of the medium (D-s( -1)) and Grunwald-Winstein values of the solvent (Y) were non-linear. The variation of enthalpies (Delta H) and entropies (Delta S*) of act ivation with solvent composition have been determined. Plots of Delta H or Delta S* versus the mole fraction of the solvent exhibit a maxim um at x(2) ca. 0.1 and a minimum of x(2) ca. 0.3; a linear plot of Del ta H versus Delta S* is obtained. Furthermore, the cycle relating the free energy of activation in H2O to that in H2O-MeOH shows that chang es in the solvent structure in H2O-MeOH mixtures generally stabilize t he five-coordinate cation in the transition state, more than the catio n in the initial state as the mole fraction of MeOH increases. The res ults are discussed and compared with other related systems.