Intramolecular Electron Transfer

In most of the oxidation—reduction reactions involving net electron transfer which have been studied, the experimental results yield only second order rates, and thus reflect the barrier to bringing reactants together as well as that for the electron transfer act itself. Recent advances in synthetic procedures make it possible to measure as an intramolecular or first order process, net electron transfer in complexes containing Co(III) and Ru(II) as oxidizing and reducing agents respectively for a wide variety of bridging groups. Systems in which the bridging groups mediate in electron transfer, as well as others, in which the bridging groups serve only to define the separation of the oxidizing agent and reducing centres become accessible to study. In a somewhat related class of reactions, a particular ligand in an oxidizing complex is converted by external means to a reducing agent, and the rate of reduction of the oxidizing metal ion by the damaged ligand is measured, as a first order process. Closely related in principle to the work referred to are studies on mixed valence complexes such as [(H3N)5Ru N( )N Ru(NH3)5]5 where electron transfer between a ruthenium (II) and a ruthenium (III) end can be considered. But the two classes differ dramatically because in the latter system there is no chemistry associated with electron transfer, in the sense that no bonds are broken or formed on electron transfer. The mixed valence complexes in fact require different experimental approaches for their study and feature elements of interest different from those exhibited by the Co(III)— Ru(II) class. The principles underlying the behaviour of the class of molecules will be outlined, and the results obtained in the study of their properties are described. INTRODUCTION Several lines of research, each guided by some specialized ideas and interests, have led to the current concern with intramolecular electron transfer. Only those subjects which have a chemical background will be dealt with here. It will be apparent, however, that at a basic level some of the issues which will be raised are identical to those which are important for understanding electronic phenomena in other fields, as for example, electronic conductance in solids. Thus, although in one sense the title subject represents a narrowing of focus in each of several fields, in the sense that it involves a confluence of ideas from these fields, it represents a broadening of the scientific horizon.