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 t erms of steric strain induced by the ligand systems. The intersections of potential energy curves (strain energy versus metal-ligand distanc e 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 in ner sphere reorganization (Delta H-double dagger, and correlated with experimentally determined electron self-exchange rates, The minima of these potential energy curves of the reduced and oxidized forms are co rrelated with the reduction potentials. The perturbation by electronic effects due to differences in nucleophilicity along the series ammoni a, primary amine, secondary amine, tertiary amine has been accounted f ar. 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 Co3+-N distances (1.94-2.05 Angstrom), and the ligand field strengths (Co3+: (1)A(1) --> T-1(1), 16 700-22 200 cm(-1)) cover a wid e range. Accurate computed values for extremely long Co3+-N bonds and for the corresponding low ligand field parameters (MM-AOM), high redox potentials, and specific electron self-exchange rates could only be o btained with a modification of the originally used force field, involv ing Morse potentials for the metal-ligand bonds. Applications of these methods, involving the design of new oxidants or reductants with spec ific potentials and electron transfer rates, and the determination of solution structures based on experimentally determined redox propertie s are presented, limits of this purely steric approach are discussed, and alternatives are evaluated.