Electron transfer binuclear complexes

H2 or O2 from water in the presence of a sacrificial reductant or oxidant employ a mthenium complex, typically [Ru(bipy)2], as the photon absorber (96,97). A series of mixed binuclear mthenium complexes having a variety of bridging ligands have been the subject of numerous studies into the nature of bimolecular electron-transfer reactions and have been extensively reviewed (99—102). The first example of this system, reported in 1969 (103), is the Creutz-Taube complex [35599-57-6] [Ru2(pyz)(NH3. 

Electron-Transfer in Simple Binuclear Complexes. In trying to understand the electron transfer mediation effects of peptide bonds and amino acid side chains on rates of electron transfer in simple systems that are amenable to detailed investigation, we have designed and synthesized a series of complexes which contain within a single molecule two different oxidizing agents —both of which are inert to substitution. The series of complexes we have synthesized is represented schematically by the general structure 1. 

The outer-sphere pathway k ) produces the final products directly, as shown by a rapid increase in absorbance at 474 nm, which is a maximum for Ru(NH3)5pz. At the same time, a rapid inner-sphere (A ,) production of the binuclear complex takes place. A slower absorbance increase at 474 nm arises from the back electron transfer in the binuclear intermediate This produces the original reactants which then undergo outer-sphere reaction (Atj). 

In a few instances, binuclear complexes have been reported in which the two copper atoms interact directly through a metal-metal bond. Two examples of such complexes are illustrated below [192, 193]. CV s run on the compound at left in both tetrahydrofuran and CH2CI2 indicated the presence of two separate electron-transfer steps that were quasi-reversible. 

Figure 9 Interfacial and remote electron-transfer process for the binuclear [fac-Re(I) (dcb)(CO)3(CN)Ru(II)(bpy)2(CN)] complex on Ti02.

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

A simpler situation from the point of view of a theoretical treatment -although more difficult to study experimentally - is electron exchange between ions which constitute two halves of the same redox couple, e.g. MnO /MnO -, Co(NH3) +/Co(NH3)6+ etc. Two distinct types of mechanism have been postulated. In the outer-sphere mechanism, the coordination spheres of both oxidant and reductant remain intact as electrons are transferred, and the oxidation numbers of the central atoms change. The inner-sphere mechanism describes a situation where a bridged binuclear complex is formed as an intermediate, and the bridging ligand - which may be Cl-, OH etc. or an ambidentate ligand like NCS" - provides a pathway for electron transfer. 

As an example, a four-electron transfer from two M+n metal atoms in a binuclear complex may be visualized ... 

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