Electron transfer bridging ligand

In the above example, Cl is transferred between metal centres such transfer is often (but not necessarily) observed. In the reaction between [Fe(CN)g] and [Co(CN)5], the intermediate 25.4 (which is stable enough to be precipitated as the Ba salt) is slowly hydrolysed to [Fe(CN)6] and [Co(CN)5(H20)] without transfer of the bridging ligand. Common bridging ligands in inner-sphere mechanisms include halides, [OH], [CN], [NCS] , pyrazine (25.5) and 4,4 -bipyridine (25.6) pyrazine acts as an electron-transfer bridge in the Creutz-Taube cation and related species (see structure 22.57 and discussion). 

An explanation for the role of the electronic configurations at the donor and acceptor sites has been developed by considering a reference state where the transferring bridging ligand is equidistant between oxidant and reductant and nonbonding, for example Cr "—X—Cr Cr, X,Cr . ... 

The oxidant can also dictate an outer-sphere mechanism. In Table 12.15, [Co(NH3)6] and [Co(en)3] " participate in outer-sphere electron transfer their ligands have no accessible lone pairs with which to form bonds to bridge with the reductant. The electron transfer mechanisms for the other reactions are less certain, although labile CT (aq) is assumed to react by inner-sphere mechanisms when bridging is possible. 

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. 

N2 recognized as a bridging ligand in ((NH3)5RuN2Ru(NH3)5] by D. F. Harrison, E. Weissterger, and H. Taute. (H. Taute, 1983 Nobel Prize for chemistry for his work on the mechanisms of electron transfer reactions especially in metal complexes ). 

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

Non-bridging ligands in electron transfer reactions. J. E. Earley, Prog. Inorg. Chem., 1970, 13, 243-262 (76). 

Role of the bridging ligand in inner-sphere electron transfer reactions. A. Haim, Acc. Chem. Res., 1975, 8, 264-272 (80). 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

In systems such as [A... A ]+ where an electron (or a hole) hesitates or oscillates between two equivalent positions on subsystems A or A, symme breakings may occur when the effective transfer integral between the two sites is weak. Hiis will be the case when A and A are far apart, when they are bridged by an "insulating" ligand, or when the two localized MOs concerned by the electron transfer have a very we spatial overlap. 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

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