Electron transfer

Electron transfer (ET) is the process by which an electron moves from an atom or a chemical species (e.g. a molecule) to another atom or chemical species. ET is a mechanistic description of the thermodynamic concept of redox, wherein the oxidation states of both reaction partners change.

Numerous biological processes involve ET reactions, including oxygen binding, photosynthesis, respiration, and detoxification routes. Additionally, the process of energy transfer can be formalized as a two-electron exchange (two concurrent ET events in opposite directions) in case of small distances between the transferring molecules. ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic chemistry.

Classes of electron transfer
There are several classes of electron transfer, defined by the state of the two redox centers and their connectivity

Inner-sphere electron transfer
In inner-sphere ET, the two redox centers are covalently linked during the ET. This bridge can be permanent, in which case the electron transfer event is termed intramolecular electron transfer. More commonly, however, the covalent linkage is transitory, forming just prior to the ET and then disconnecting following the ET event. In such cases, the electron transfer is termed intermolecular electron transfer. A famous example of an inner sphere ET process that proceeds via a transitory bridged intermediate is the reduction of [CoCl(NH3)5]2+ by [Cr(H2O)6]2+. In this case the chloride ligand is the bridging ligand that covalently connects the redox partners.

Outer-sphere electron transfer
In outer-sphere ET reactions, the participating redox centers are not linked via any bridge during the ET event. Instead, the electron "hops" through space from the reducing center to the acceptor. Outer-sphere ET is by definition intermolecular. Outer sphere electron transfer can occur between different chemical species or between identical chemical species that differ only in their oxidation state. The later process is termed self-exchange. As an example, self-exchange describes the degenerate reaction between permanganate and its one-electron reduced relative manganate:


 * [MnO4]- + [Mn*O4]2-  →  [MnO4]2- +  [Mn*O4]-

In general, if electron transfer is faster than ligand substitution, the reaction will follow the outer-sphere electron transfer.

Often occurs when one/both reactants are inert or if there is no suitable bridging ligand.

A key concept of Marcus theory is that the rates of such self-exchange reactions are mathematically related to the rates of "cross reactions". Cross reactions entail partners that differ by more than their oxidation states. One example (of many thousands) is the reduction of permanganate by iodide to form iodine and, again, manganate.

Five steps of an outer sphere reaction

 * 1. reactants diffuse together out of their solvent shells => precursor complex (requires work =wr)
 * 2. changing bond lengths, reorganize solvent => activated complex
 * 3. Electron transfer
 * 4. Relaxation of bond lengths, solvent molecules => successor complex
 * 5. Diffusion of products (requires work=wp)

Heterogeneous electron transfer
In heterogeneous electron transfer, an electron moves between a chemical species and a solid-state electrode. Theories addressing heterogeneous electron transfer have applications in electrochemistry and the design of solar cells.

Theory
The first generally accepted theory of ET was developed by Rudolph A. Marcus to address outer-sphere electron transfer and was based on a transition-state theory approach. The Marcus theory of electron transfer was then extended to include inner-sphere electron transfer by Noel Hush and Marcus. The resultant theory, called Marcus-Hush theory, has guided most discussions of electron transfer ever since. Both theories are, however, semiclassical in nature, although they have been extended to fully quantum mechanical treatments by Joshua Jortner, Alexender M. Kuznetsov, and others proceeding from the Fermi's Golden Rule and following earlier work in non-radiative transitions. Furthermore, theories have been forwarded to take into account the effects of vibronic coupling on electron transfer. In particular the PKS theory of electron transfer.

Before 1991, ET in metalloproteins was thought to affected primarily be the average, diffuse properties of the non-metal atoms forming an insulated barrier between the metals, but Beratan, Betts and Onuchic subsequently showed that the ET rates are governed by the bond structures of the proteins, that the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins.