260-267The kinetics of intramolecular electron transfer between COIIl and SIV in O-sulphitopentaamminecobalt(III), (NH3)5 CoOSO+2, have been investigated in aquo-organic solvent media comprising dipolar protic cosolvents (methanol, n-propanol, isopropanol, tert-butanol, and ethyleneglycol) and dipolar aprotic cosolvents (acetonitrile, ethylene carbonate) at 25-40°C. Rate acceleration is generally observed with increasing mole fraction (Xorg) of the cosolvent except for ethyleneglycol + water. Correlation of rate (In ksred) with Xorg GE (Gibbs excess free energy), ENT and dielectric constants of the solvent mixtures using multiparameter regression indicates that preferential solvation and solvent structural effects are important in controlling the reactivity. The correlation of rate with solvent hydrogen bonding acidity (α), basicity (β) and polarizability (π*)for acetonitrile+water {ln ksred=(2.5±0.4)-(2.0±0.6)α +(0.2±0.4)β-4.0±0.4)π*} indicates that the electron transfer rate conformed well with the Taft model of solvation. The activation enthalpy and entropy versus Xorg plots display extrema at low Xorg,∆.H+ versus ∆S+ plot is linear indicating that the solvent structure effects on these parameters are mutually compensatory
