outersphere

Outersphere, in chemistry and electrochemistry, refers to a mode of electron transfer where the donor and acceptor exchange electrons without forming a direct chemical bond or a bridging ligand between them. In an outer-sphere transfer, the reactants retain their primary coordination environments before and after the event, and the transfer occurs through space or via the surrounding solvent rather than through a shared ligand.

Mechanistically, outer-sphere electron transfer relies on electronic coupling between the donor and acceptor and on the reorganization of the solvent and the internal structures of the species involved. The rate is commonly analyzed with theories such as Marcus theory, which relates the rate to the driving force of the reaction and the reorganization energy required to accommodate the charge rearrangements. Factors that influence outer-sphere rates include the distance between redox centers, solvent polarity, and the molecular orientation of the participating species.

Outer-sphere transfers are contrasted with inner-sphere mechanisms, where a transient bond or bridging ligand forms a pathway for electron movement. In inner-sphere processes, changes to the bridging ligand or the coordination sphere play a direct role in the electron transfer event. Outer-sphere reactions typically exhibit less dependence on specific ligand bridges and can occur even when the coordination environments of the species are well separated.

Common examples include aqueous redox couples such as ferro-/ferricyanide and certain metal–ligand complexes in solution. Understanding outer-sphere electron transfer is important for electrochemical devices, corrosion processes, and various chemical sensors where direct bonding between redox partners is unlikely.