An outer-sphere electron transfer is a reaction in which the electron transfer takes place with no or very weak (4-16 kJ mol-1) electronic interaction between the reactants in the transition state. If instead the donor and the acceptor exhibit a strong electronic coupling, the reaction is described as inner-sphere electron transfer. The two terms derive from studies concerning metal complexes and it has been suggested that for organic reactions the terms 'nonbonded' and 'bonded' electron transfer should be used.