Electron-transfer reactions
Electron transfer (ET) occurs when an electron relocates from an atom or molecule) to another such chemical entity. ET is a mechanistic description of a redox reaction, wherein the oxidation state of reactant and product changes.
Numerous biological processes involve ET reactions. These processes include oxygen binding, photosynthesis, respiration, and detoxification. 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.
There are several classes of electron transfer, defined by the state of the two redox centers and their connectivity
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.
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 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:
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