Electron transfer in mixed-valence biferrocenium salts: effect of zero-point energy difference and pronounced anion dependence

Abstract

that the photoreaction is strongly inhibited in the en complex and that nonradiative decay to the ground state is a more effective competitor. Part of this rate inhibition in the en complex could arise prior to reverse intersystem crossing because the larger energy barrier for the activated route allows more competitive doublet nonradiative decay. However, the extent to which such decay can be accommodated is restricted by the observed linearity of the Arrhenius plot. At the transition-state level, other possible reasons are easy to imagine. If the transition state is seven-coordinate, then it may be more sterically crowded and have higher strain energy for the en complex than for the diammine. Alternatively, it may be that the required migration of the whole en ligand is a difficult and slow process so that deactivation and/or recoordination have a greater opportunity to compete. Finally, the diammine complex reactsz7 via a seven-coordinate species of microsecond lifetime. If the en complex behaves similarly, then its intermediate might be more likely to revert to starting material because of the bidentate nature of the leaving ligand. Concluding Remarks. We have presented evidence that after photochemical Cr-N bond breaking has occurred, it is possible for the reacting bidentate ligand to migrate from one coordination site to another in an edge displacement reaction. Since our proof relies on the assumption of stereochemical change, in the future additional evidence will be sought. It is fairly straightforward to design experiments with appropriate stereochemical sign-posting to prove group migration incontrovertably. Unfortunately, it is difficult to identify systems in which the starting compound can be prepared and unambiguously characterized and its photochemistry explored with product identification including stereochemistry. The work has shown, however, that complex ligand motions are likely and that further study of such phenomena is tractable and worth pursuing.