Electron Transfer Among the Transition Elements; the Controlling Role of the Franck—Condon Principle on Rates

Abstract

The principle [W. F. Libby, J. Phys. Chem. 56, 863 (1952)] that electrons cannot be readily exchanged between aqueous ions of different valence, unless they possess sufficient geometrical similarity to reduce to a minimum the energy transfer required by the simultaneous and instantaneous conversion of an ion of one valence to another while a second is changed in the exact opposite way, has now been firmly established. The Franck—Condon principle as applied to electron movement as well as to optical transitions thus has been found to be completely commanding in the case of electron exchange reactions between transition element ions in aqueous solution; e.g., Fe2+ and Fe3+ exchange electrons slowly while Fe(CN)64+ and Fe(CN)63— exchange very rapidly. Rapid electron exchange occurs also between MnO4—— and MnO4— even though the oxygen atoms do not exchange. Thus a type of tunneling of electrons is firmly proved to occur when the source and sink (reducing and oxidizing) ions (or molecules) satisfy the requirement of the Franck—Condon principle of being sufficiently similar geometrically to leave little need for change in hydration energy after the electron transfer. It seems clear that this principle should apply also to oxidation and reduction reactions among the transition elements, and the present work examines the available kinetic data (particularly those of Taube and co-workers) to establish the extent of this applicability which appears to be substantial.