In outer-sphere ET reactions, the participating redox centers are not linked via any bridge during the ET event.  Instead, the electron "hops" through space from the reducing center to the acceptor.  Outer sphere electron transfer can occur between different chemical species or between identical chemical species that differ only in their oxidation state.  The latter process is termed self-exchange.  As an example, self-exchange describes the degenerate reaction between permanganate and its one-electron reduced relative manganate:
:[MnO] + [Mn*O] → [MnO] + [Mn*O]
In general, if electron transfer is faster than ligand substitution, the reaction will follow the outer-sphere electron transfer.
Often occurs when one/both reactants are inert or if there is no suitable bridging ligand.
A key concept of Marcus theory is that the rates of such self-exchange reactions are mathematically related to the rates of "cross reactions".  Cross reactions entail partners that differ by more than their oxidation states.  One example (of many thousands) is the reduction of permanganate by iodide to form iodine and, again, manganate.