Initial state and transition state contributions to solvent effects on activation enthalpies for substitution reactions of tris(1,10-phenanthroline)iron(II) in aqueous methanol

Abstract

Activation enthalpies have been determined for aquation of thetris(1,10-phenanthroline)iron(II) cation and for its reaction with hydroxide in 20, 40, and 80% (by volume) methanol, and for its reaction with cyanide in 20 and 40% methanol. Enthalpies of solution of the chloride salt of this cation, and of potassium cyanide, have been determined in the water-methanol mixtures. From these results and ancillary thermodynamic data we have dissected the variations of activation enthalpies for these three substitution reactions with solvent composition into their initial state and transition state components. In the water-rich mixtures (0 to 40% methanol) initial and transition state solvation effects are similar and dominated by the ligands, but in the reaction with hydroxide in 80% methanol there is a striking difference between initial and transition state behaviour.