Fast scan rate cyclic voltammetry has been used to elucidate the heterogeneous and homogeneous aspects of the redox chemistry of cis- and trans- [Mn(CN)(CO)(2){P(OPh)(3)} (Ph2PCH2PPh2)]. The cis species, on oxidation, isomerizes to the trans conformation, and the various processes can be understood as components of a square reaction scheme. In solution phase, the homogeneous rate constant for isomeric conversion is found to be independent of solvent and concentration, with a value of 3 8 s(-1) obtained by comparison with digitally simulated responses. The heterogeneous rate constants for the cis(0)/cis(+) and trans(0)/trans(+) couples in acetonitrile have values of 0.06 and 0.065 cm s(-1) respectively. The cis or trans complex was readily immobilized on an electrode surface, either by mechanical attachment or by precipitation from solution. However, electrolysis in the solid state was strongly dependent on the identity of the supporting electrolyte anion. This dependence is similar to the Hofmeister series of anions with large, hydrophobic ions favored in the charge-neutralization process. The addition of acetonitrile to the aqueous electrolyte solution dramatically increases the magnitude of the voltammetric peak currents but only in the presence of these ions. The effect is reversible, and other organic solvents have a similar effect. Infrared measurements conducted on the solid immobilized on glassy carbon before and after electrolysis clearly demonstrate the occurrence of the cis-trans isomerization in the solid state. The isomerization rate is considerably slower in the solid matrix relative to that in solution phase because of the restriction of free space; voltammetric measurements conducted on the solid cis complex immobilized on an electrode surface reveal a rate constant of 2 s(-1). The solvent independence of the isomerization rate in solution phase and the fact that the reaction is observed with an appreciable rate in the solid state suggest that the conformational change occurs via a nondissociative twist mechanism. Morphological changes accompanying the solid-state redox electrochemistry, observed with both thin and thick coatings of microparticles, were evaluated by electron scanning microscopy and revealed the formation of needlelike crystals on the electrode surface during the course of potential cycling experiments.