Outer-sphere mechanism When [Fe(phen)3]3+ (phen is orthophenanthroline) is reduced by [Fe(CN)6]4-, no ligand bridge forms between the metals and an electron moves from the HOMO of Fe(II) to the LUMO of Fe(III) in a very short and direct contact between the two complexes. As the result of the electron transfer, [Fe(phen)3]2+ and [Fe(CN)6]3- form. This kind of reaction is a redox one via the outer-sphere mechanism, and is characteristic of a complex system that has a very slow ligand substitution rate compared with the speed of electron transfer, especially in systems that have the same ligands but different oxidation-numbers, for example, [Fe(CN)6]3- - [Fe(CN)6]4- has a high rate of electron transfer. R. A. Marcus won the Nobel Prize (1992) for his study of this outer-sphere electron transfer mechanism.