Intramolecular Electron Transfer Reactions of Ion Pairs: Thermal, Optical, and Photochemical Pathways

It has been recognized for several years that bimolecular electron transfer reactions1 (as well as other reactions2) proceed via a sequence of elementary steps, namely, formation of the precursor complex, intramolecular electron transfer within the precursor complex, and dissociation of the successor complex. For outer-sphere reactions, the precursor and successor complexes are ion pairs or outer-sphere complexes. For inner-sphere reactions, the precursor and successor complexes are binuclear complexes in which a bridging ligand connects the two metal centers. Under conditions where the stability of the precursor complex is low (Q IP or Qp « 1) and the electron transfer step ket is rate determining, experimental kinetic measurements yield second-order rate constants k exp which are equal to the product of the equilibrium constant for the formation of the precursor complex and the rate constant for electron transfer, k exp=Q IP k et or Qpk et.3 The overwhelming majority of electron transfer reactions of transition metal complexes-both outer-sphere and inner-sphere reactions-conform to the above trend, and under these circumstances kinetic measurements do not yield direct information about the act of electron transfer itself.