Historically an @E02011@ between two metal centres sharing a ligand or atom in their respective @C01329@ shells. The definition has more recently been extended to any situation in which the interaction between the donor and acceptor centres in the @T06468@ is significant (\(> 20\ \text{kJ mol}^{-1}\)).
See also:
outer-sphere electron transfer
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1125 [Terms] [Paper]
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2247 [Terms] [Paper]