A chemical reaction that involves the transfer, addition, or removal of electrons. Electron-transfer reactions often involve complexes of transition metals. In such complexes one general mechanism for electron transfer is the inner-sphere mechanism, in which two complexes form an intermediate, with ligand bridges enabling electrons to be transferred from one complex to another complex. The other main mechanism is the outer-sphere mechanism, in which two complexes retain all their ligands, with electrons passing from one complex to the other.

The rates of electron-transfer reactions vary enormously. These rates can be explained in terms of the way in which molecules of the solvent solvating the reactants rearrange so as to solvate the products in the case of the outer-sphere mechanism. In the case of the inner-sphere (ligand-bridged) reactions the rate of the reaction depends on the intermediate and the way in which the electron is transferred. The theory of electron-transfer reactions is sometimes called Marcus theory after the Canadian-born US chemist Rudolph Marcus (1923– ), who led its development in the 1950s.