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The rate constants for the solvolysis of trans-[Cr(en)2Br2]+ ion were determined by the spectrophotometric method in methanol-, ethanol-, acetone-, and acetonitrile-water mixtures, at 20, 25, 30, and 35℃, respectively. The rate constants increased with increasing co-solvent compositions. The rate constant did not show any relation with the reciprocal of dielectric constant of the solvent-mixtures. The m values of Grunwald-Winstein equation for methanol-, ethanol-, acetonitrile-, and acetone-water mixtures are 0.109, 0.103, 0.101, and 0.095, respectively. A free energy cycle for the process from the initial state to the transition state in water and water + co-solvent mixtures shows that the change in solvation at the transition state has a dominant effect on the rate. From the above results, it is believed that the mechanism for the aquation of this complex is the Id mechanism.
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