Metal complexes—continued oxidation-reduction reactions

The nature and properties of metal complexes have been the subject of important research for many years and continue to intrigue some of the world s best chemists. One of the early Nobel prizes was awarded to Alfred Werner in 1913 for developing the basic concepts of coordination chemistry. The 1983 Nobel prize in chemistry was awarded to Henry Taube of Stanford University for his pioneering research on the mechanisms of inorganic oxidation-reduction reactions. He related rates of both substitution and redox reactions of metal complexes to the electronic structures of the metals, and made extensive experimental studies to test and support these relationships. His contributions are the basis for several sections in Chapter 6 and his concept of inner- and outer-sphere electron transfer is used by scientists worldwide. 

The coverage in this chapter is not comprehensive and no tabulation of rate data has been attempted. A more extensive account of the material will appear in Volume 9 of the series the presentation of the material also differs slightly from previous volumes. There have been many important developments in this general area. A comprehensive review of one-electron reduction potentials for nonmetallic substrates has appeared. Detailed kinetic studies have been reported for several important reactions where metal ions serve as catalysts in the transformation of organic substrates.Catalytic oxidation reactions involving metal complexes and macrocyclic metal complexes have been reviewed. Continued interest centers on catalysis by metalloporphyrins, and the role of metal complexes in electrocatalytic reductions has been reviewed. ... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

The corrosion inhibitor can be a complexing agent that stops the metal redeposition reaction (reduction) by eliminating free metal ions from the solution. Theoretically, this would consecutively stop the associated oxidation reaction. Due to parasitic reduction reactions, however, the metal oxidation can continue, even enhanced by the complexing agent effect. 

Bipyridyl (continued) as ligand, 12 135-1% catalysis, 12 157-159 electron-transfer reactions, 12 153-157 formation, dissociation, and racemization of complexes, 12 149-152 kinetic studies, 12 149-159 metal complexes with, in normal oxidation states, 12 175-189 nonmetal complexes with, 12 173-175 oxidation-reduction potentials, 12 144-147... 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

Corrosion inhibition Is a very important but still poorly understood process. The aim is to inhibit the continuous oxidation or reduction of metals. This requires insight into the electrochemistry of the process. A host of interwoven phenomena are involved, including redox reactions on the surface, in the solution and Intervention by inorganic or organic molecules. The complexity is illustrated by the fact that there is often no simple correlation between the coverage of a certain inhibitor on the surface and the extent of Inhibition. 

In the case of transition metal complexes, their activity as homogeneous catalysts is well documented, but here the purpose is immobilization in a solid that should made possible the easy recovery of the complex from the reaction mixture and its reuse or operation under continuous flow. Immobilization has the benefit of avoiding complex dimerization and aggregation that is one of the common deactivation pathways for metallic complexes as homogeneous catalysts in many types of reactions, including oxidations and reductions. 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

Although the main applications of zeohtic sohds in catalysis will continue to be as solid acids in the synthesis and transformations of petrochemicals and commodity chemicals they continue to be considered as catalysts and catalyst supports for a range of reactions of synthetic and industrial relevance. The most important of these are of titanium- and tin-containing solids in selective oxidations. Other well-studied reactions over zeohtes include light hydrocar-bons-to-aromatics (Ga-zeolites) selective catalytic reduction of NO (transition metal exchanged zeolites) C C bond formation (Pd zeohtes) selective alkane oxyfunctionalisation with air (MAPOs, M Mn, Fe, Co) and chiral catalysis over encapsulated chiral complexes. 

Metal-ion reduction of both mononuclear and dinuclear carboxylatocobalt(m) complexes have been reviewed. There continues to be substantial interest in this subject and in particular the adjacent-attack mechanism for complexes with simple monocarboxylate ligands now appears to be well understood. The importance of the iimer-sphere mechanism in reductions by Eu + has been amply demonstrated. Gould has also illustrated the usefulness of the comparison of rate data from the reactions of a common oxidant by several reductants. Many workers are currently involved in attempts to measure first-order rates of electron transfer within precursor complexes. In the search for likely systems, the one chosen by Taube ... 

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