Electron transfer

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 oxidation states of both reaction partners change.

Numerous processes in biology 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). ET reactions commonly involve transition metal complexes, [Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.] [Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.] 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 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 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.

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