Recent Progress in the Study of Inner-sphere Electron Transfer Reactions

Progress made during the past ten years or so by the author's associates in studying inner-sphere electron transfer reactions is reviewed, emphasizing organic molecules as bridging groups. Evidence is outlined for reduction by remote attack in the reaction of chromous ion with isonicotinamidepentaamminecobalt(ni) and with the analogous nicotinamide complex. That there be a conjugated bond system in a bridging group, even when there is a suitable remote polar group, is not a sufficient condition for effective mediation by the bridging group in electron transfer by remote attack. Moreover, conjugated bond systems are shown to exert effects even when they are not in the direct line of electron transfer, that is, when they are in pendent positions. The reducibility' of the bridging group appears to be important in determining its capacity to mediate in electron transfer between Cr(Ii) and Co(m). In the systems referred to, with reducible bridging ligands, electron transfer is understood as taking place in a stepwise manner. As was anticipated by Halpern and Orgel, the symmetry properties of the donor, carrier and acceptor orbitals are important in determining rates and mechanisms. Thus the mechanism of electron transfer seems to be different when Ru(m), a it electron acceptor, rather than Cr(ni) or Co(m), which are a acceptors, is oxidant. The electronic structures of the metal ions are important also in determining the lability of binuclear precursor and product complexes. A number of product binuclear complexes are mentioned containing both Ru(II) and Cr(m) bound to a bridging group, and evidence for an electromeric equilibrium in such systems is cited. The study of the binuclear complexes is an interesting subject in its own right, and the ion (XXI) (see text) is introduced as illustrating some of the issues relating to the interaction between metal ions which arise in considering binuclear species. Finally, some of the opportunities and problems facing those interested in the field are outlined. IN a plenary lecture given a decade ago' during the Fifth International Conference on Coordination Chemistry, a subject which was then still very new, that of electron transfer between metal ions as mediated by organic ligands, was featured prominently. Much additional research on this topic has been done in the past ten years, and there have been basic changes in the way it is understood. The field seems to me to be important enough to have warranted the large expenditure of effort which has been devoted to it. Most of the research of my own group in the field of electron transfer reaction has been done on systems reacting by inner-sphere mechanisms, and concern with organic molecules as bridging ligands has been central to it. In reviewing