Electron transfer (ET) is the process by which an electron moves from one atom or molecule to another atom or molecule. ET is a mechanistic description of the thermodynamic concept of redox, wherein the formal oxidation states of both reaction partners change. Numerous essential processes in biology employ ET reactions, including: oxygen binding/transport, photosynthesis/respiration, metabolic syntheses, and detoxification of reactive species. Additionally, the process of energy transfer can be formalized as a two electron exchange (two concurrent ET events in opposite directions). ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic molecules.
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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 connected to each other 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+, as described by Taube. In this case the chloride ligand is the bridging ligand that 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 differing 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]-
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.
Heterogeneous electron transfer
In heterogeneous electron transfer an electron is moved 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 Rudolph A. 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.
People
The theory of electron transfer has been extensively studied by chemists and physicists. Some notable chemists who have contributed to the theory of electron transfer include Rudolph A. Marcus, Noel Hush, Henry Taube, Norman Sutin, Bruce S. Brunschwig, Marshall Newton, Joshua Jortner, Sheng H. Lin, David N. Beratan, David Chandler, Mark Ratner, Alexender M. Kuznetsov, Uri Peskin and Gunter Grampp. Physicists who have contributed to this theory include Revaz Dogonadze. Important experimental work has been carried out by the groups of Gerhard Closs, Harry Gray, Bruce S. Brunschwig, Carol Creutz, Don Devault and many others.
