Multi-oxidation-state metals

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

Redox propcTties of some oxo-bridged di- and tri-nuclear complexes of the second and third transition elements. Re, Ru, Os, and Mo, are discussed with special attention to stability of the mixed-valence states. These complexes display multi-step one-election transfer processes and give fairly stable mixed-valence oxidation state. Stability of the mixed-valence states is discussed in terms of qualitative molecular orbital schemes based on dJt (metal) - prt (bridged oxygen) interactions. 

Polyoxometalates (POMs) are discrete molecular structures composed of metal cations bridged by oxide anions. They are not multi-metal species with metal-metal bonding, one conventional definition of clusters, but they are clusters in the generic sense of the word. A number of general reviews on POMs have appeared in recent years.1-13 Since a great majority of these structures are anionic, and thus salts with obligatory cations, and constituted primarily of early-transition-metal (d-block) elements in their highest oxidation states, the... 

To achieve the required reductions in metal ion concentrations, it may be necessary to carry out a multi-stage treatment. In other cases, the treatment may require the oxidation state of the metal to be changed (e.g., oxidation of ferrous iron to ferric, which is more readily removed at low pH levels, and reduction of soluble Cr(VI) to Cr(III), which can readily be precipitated). 

Multi-mode molecular switching properties and functions of azo-conju-gated terpyridine complexes of transition metals have been studied [47-49]. The dependences of the photoisomerization behavior on the metal center and its oxidation state were investigated using azobenzene-attached terpyridine complexes of four metals Fe, Co, Ru, and Rh. As for the Fe and Ru complexes, 60-62, photoisomerization is totally inhibited when UV light irradiation excites the n -n band of the azobenzene moiety in the trans form due to the occurrence of energy transfer from the azobenzene unit to the complex unit [50-53]. 

Ion chromatography can also be applied for ultra-trace analysis of transition metals. In comparison to ICP-MS it offers the advantageous ability to specify the oxidation state of metals and, moreover, to carry out multi-element determinations in the lowest ng/L range after pre-concentration. Transition metal separation is performed with ion exchangers that have defined anion and cation exchange capacities detection is carried out photometrically after derivatization... 

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