Metal ion-coupled and decoupled electron transfer

Effects of metal ions on thermal and photoinduced electron-transfer reactions from electron donors (D) to electron acceptors (A) are reviewed in terms of metal ion-coupled electron transfer (MCET) vs. metal ion-decoupled electron transfer (MDET). When electron transfer from D to A is coupled with binding of metal ions to A^?, such an electron transfer is defined as MCET in which metal ions accelerate the rates of electron transfer. A number of examples of electron-transfer reactions from D to A, which are energetically impossible to occur, are made possible by strong binding of metal ions to A^? in MCET. The structures of metal ion complexes with A^? are also discussed in relation with the MCET reactivity. The MCET reactivity of metal ions is shown to be enhanced with an increase in the Lewis acidity of metal ions. In contrast to MCET, strong binding of metal ions to A^? results in deceleration of back electron transfer from metal ion complexes of A^? to D⁺ in the radical ion pair, which is produced by photoinduced electron transfer from D to A in the presence of metal ions, as compared with back electron transfer without metal ions. The deceleration of back electron transfer in the presence of metal ions results from no binding of metal ions to A. This type of electron transfer is defined as metal ion-decoupled electron transfer (MDET). The lifetimes of CS state (D⁺–A^?) produced by photoinduced electron transfer from D to A in the D–A linked systems are also elongated by adding metal ions to the D–A systems because of the stabilization of the CS states by strong binding of metal ions to A^? and the resulting slow MDET processes.