Electron transfer (ET) occurs when anelectron relocates from anatom,ion, ormolecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred inredox reactions.[2]
Electrochemical processes are ET reactions. ET reactions are relevant tophotosynthesis andrespiration and commonly involvetransition metal complexes.[3][4] Inorganic chemistry ET is a step in some industrial polymerization reactions. It is foundational tophotoredox catalysis.
In inner-sphere ET, 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+.[5][6] In this case, the chlorideligand is the bridging ligand that covalently connects the redox partners.[7]
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 latter process is termed self-exchange. As an example, self-exchange describes thedegenerate reaction betweenpermanganate and its one-electron reduced relativemanganate:
In general, if electron transfer is faster than ligand substitution, the reaction will follow the outer-sphere electron transfer route.
Outer-sphere ET reactions often occur when one/both reactants are inert or if there is no suitable bridging ligand.
A key concept ofMarcus theory[8] 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 byiodide to formiodine and manganate.
In heterogeneous electron transfer, an electron moves between a chemical species present insolution and the surface of a solid such as asemi-conducting material or anelectrode. Theories addressing heterogeneous electron transfer have applications inelectrochemistry and the design ofsolar cells.
Especially in proteins, electron transfer often involves hopping of an electron from one redox-active center to another one. The hopping pathway, which can be viewed as avector, guides and facilitates ET within aninsulating matrix. Typical redox centers areiron-sulfur clusters, e.g. the 4Fe-4Sferredoxins. These sites are often separated by 7-10 Å, a distance compatible with fast outer-sphere ET. It has been found that the matrix of ET proteinplastocyanin (devoid of the redoxcopper ion) is sufficient to support charge transport with itsredox partnerphotosystem I.[9]
The first generally accepted theory of ET was developed byRudolph A. Marcus (Nobel Prize in Chemistry in 1992)[8] to addressouter-sphere electron transfer and was based on atransition-state theory approach. The Marcus theory of electron transfer was then extended to includeinner-sphere electron transfer byNoel Hush and Marcus. The resultant theory,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 fullyquantum mechanical treatments byJoshua Jortner,Alexander M. Kuznetsov, and others proceeding fromFermi's golden rule and following earlier work innon-radiative transitions. Furthermore, theories have been put forward to take into account the effects ofvibronic coupling on electron transfer, in particular, thePKS theory of electron transfer.[10] In proteins, ET rates are governed by the bond structures: the electrons, in effect, tunnel through the bonds comprising the chain structure of the proteins.[11]
