Oxidation-reduction reactions Bridging ligand

Oxidation-reduction reactions of transition metal complexes, like all redox reactions, involve the transfer of an electron from one species to another—in this case, from one complex to another. The two molecules may be connected by a common ligand through which the electron is transferred, in which case the reaction is called a bridging or inner-sphere reaction, or the exchange may occur between two separate coordination spheres in a nonbridging or outer-sphere reaction. 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

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. 

Controlled O2 oxidation of the MoIII,ni IV,lv tetramer yields a product which titrates for MoIV and from an incomplete X-ray crystal study appears to be basically the same tetrameric structure, except that all the bridging ligands are now fi-oxo. This Mo tetramer is also an intense red colour, Figure 10. The reduction potential for the couple involving the two tetrainers is +0.07 V. Rapid reactions of both tetramers with 02 are of interest in that there is no site for binding unless the coordination number increases. Mild oxidants such as [Co(NH3)5Cl]2+ react rapidly (fi/2 < 1 min) with both tetramers to give the Mo)1 dimer. 

A related example of inner-sphere reaction is shown in reaction (5), where an additional mechanistic subtlety appears.10 As shown by the products, inner-sphere electron transfer also occurs but now by remote attack in which the sites of bridging ligand binding to the reductant and oxidant are at different atoms on the bridging ligand. 

Inner-sphere reactions also use the tunneling phenomenon, but in this case a single ligand is the conduit. The reactions proceed in three steps (1) a substitution reaction that leaves the oxidant and reductant linked by the bridging ligand, (2) the actual transfer of the electron (frequently accompanied by transfer of the ligand), and (3) separation of the products ... 

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