Modeling of the Redox Properties of (Hexaamine)cobalt(III/II) Couples.

The thermodynamics (redox potentials) and kinetics (electron transfer rates) of (hexaamine)cobalt(III/II) redox couples are interpreted in terms of steric strain induced by the ligand systems. The intersections of potential energy curves (strain energy versus metal-ligand distance plots of pairs of conformers) of the oxidized and reduced forms of a wide range of (hexaamine)cobalt(III/II) couples are related to the inner sphere reorganization (DeltaH()), and correlated with experimentally determined electron self-exchange rates. The minima of these potential energy curves of the reduced and oxidized forms are correlated with the reduction potentials. The perturbation by electronic effects due to differences in nucleophilicity along the series ammonia, primary amine, secondary amine, tertiary amine has been accounted for. The redox potentials of the couples studied (E degrees = -0.6V to +0.8 V; vs SHE), the electron self-exchange rates (10(-)(7)s(-)(1)-10(3)s(-)(1)), the Co(3+)-N distances (1.94-2.05 Å), and the ligand field strengths (Co(3+): (1)A(1) -->( 1)T(1), 16 700-22 200 cm(-)(1)) cover a wide range. Accurate computed values for extremely long Co(3+)-N bonds and for the corresponding low ligand field parameters (MM-AOM), high redox potentials, and specific electron self-exchange rates could only be obtained with a modification of the originally used force field, involving Morse potentials for the metal-ligand bonds. Applications of these methods, involving the design of new oxidants or reductants with specific potentials and electron transfer rates, and the determination of solution structures based on experimentally determined redox properties are presented, limits of this purely steric approach are discussed, and alternatives are evaluated.