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 which can be broken down into two concurrent ET events. ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic molecules.

Classes of electron transfer

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There are two main classes of ET: Inner-sphere electron transfer and Outer-Sphere electron transfer. In Inner-Sphere ET the two redox centers are covalently connected to each other during the ET. This bridge can be permament, 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 by [Cr(H₂O)₆²⁺, as described by Taube. In this case the chloride ligand is the bridging ligand that connects the redox partners.

In outer sphere ET reactions, the participating redox centers are not linked via any bridge durring 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 and its one-electron reduced relative manganate:

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A key concept of Marcus theory is that the rates of such self-exchange reactions can be 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.

Theory

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The first generally accepted theory of ET was developed by Rudolph A. Marcus to address Outer-Sphere electron transfer. Marcus's theory was then extended to include Inner-sphere electron transfer by Noel Hush. The resultant theory, called Marcus-Hush Theory, has guided all discussionsof electron transfer ever since. Both theories are, however, semiclassical in nature, although they have been extended to fully quantum mechanical treatments. Furthermore, theories have been forwarded to take into account the effects of vibronic coupling of electron transfer. In particular the PKS theory of electron transfer.

People

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The theory of electron transfer has extensively studied by chemists and physicists. Some notable chemists who have contributed to the theory or electron transfer include Rudolph A. Marcus, Noel Hush, Henry Taube, Harry Gray, Norman Sutin, and Bruce Brunschwig. Physicist who have contributed to this theory include Revaz Dogonadze.