ELECTRON-TRANSFER BETWEEN AQUAPENTAAMMINECOBALT(III) AND SULFUR(IV) IN AQUO-ORGANIC SOLVENT MEDIA - DELINEATION OF SOLVENT EFFECTS

The kinetics of intramolecular electron transfer between Co-III and SI V in O-sulphitopentaammine-cobalt(III), (NH3)(5)CoOSO2+, have been inv estigated in aquo-organic solvent media comprising dipolar protic coso lvents (methanol, n-propanol, isopropanol, tert-butanol, and ethyleneg lycol) and dipolar aprotic cosolvents (acetonitrile, ethylene carbonat e) at 25-40 degrees C. Rate acceleration is generally observed with in creasing mole fraction (X-org) of the cosolvent except for ethylenegly col + water. Correlation of rate (ln k(red)(s)) with X-org, GE (Gibbs excess free energy), E-T(N) and dielectric constants of the solvent mi xtures using multiparameter regression indicates that preferential sol vation and solvent structural effects are important in controlling the reactivity. The correlation of rate with solvent hydrogen bonding aci dity (alpha), basicity (beta) and polarizability (pi) for acetonitril e + water {ln K-red(s) = (2.5 +/- 0.4) - (2.0 +/- 0.6)alpha + (0.2 +/- 0.4)beta - 4.0 +/- 0.4)pi} indicates that the electron transfer rate conformed well with the Taft model of solvation. The activation entha lpy and entropy versus X-org plots display extrema at low X-org, Delta H double dagger versus Delta S double dagger plot is linear indicatin g that the solvent structure effects on these parameters are mutually compensatory.