Mechanistic Information from the First Volume Profile Analysis for Intramolecular Electron-Transfer Reactions:  Tetraammine−Ruthenium(Ligand) Complexes of Cytochrome c

The kinetics and thermodynamics of a series of reversible intramolecular electron-transfer reactions in systems of the type trans-(NH3)4(L)Ru(His33)−cyt c(hh) and trans-(NH3)4(L)Ru(His39)−cyt c(Ck), where L represents NH3, isonicotinamide, 4-ethylpyridine, 3,5-lutidine, and pyridine, were studied as a function of pressure in order to construct the first complete volume profiles for such processes. The volume profiles demonstrate a significant partial molar volume increase associated with the reduction of the ruthenium center. In contrast to earlier results on a series of intermolecular reactions involving cytochrome c and the corresponding pentaammine complexes, for which the volume profiles are completely symmetrical (Inorg. Chem. 1996, 35, 1564), the studied intramolecular reactions exhibit asymmetric volume profiles. The overall volume changes can be accounted for in terms of electrostriction effects centered around the ammine ligands on the ruthenium center. Explanations in terms of electronic and nuclear factors are offered to account for the asymmetrical nature of the volume profile.