Kinetic and thermodynamic study of the mechanism of reversible intermolecular electron‐transfer reactions between horse heart cytochrome c and a series of cobalt imine complexes

The Kinetics and Thermodynamics of the Reversible Outer‐Sphere Electron‐Transfer Reactions between Horse Heart Cytochrome c II/III and [Co(phen) 3 ] 3+/2+ and [Co(bpy) 3 ] 3+/2+ were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one another.