Oxidation states of metals

Most metal carbonyls are synthesized in nonaqueous media. Reactive metals, such as sodium (85), magnesium (105), zinc (106), and aluminum (107,108), are usually used as reducing agents. Solvents that stabilize low oxidation states of metals and act as electron-transfer agents are commonly employed. These include diethyl ether, tetrahydrofiiran, and 2-methoxyethyl ether (diglyme). 

To summarize, an evaluation of the oxidation state of metals in an environment is central to determining their probable fate and biological significance. Redox reactions can lead to orders of magnitude changes in the concentration of metals in various phases, and hence in their mode and rate of transport. While equilibrium calculations are a valuable tool for understanding the direction in which changes are likely to occur, field measurements of the concentrations... 

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

However, the mechanism for any particular ion may be more complex involving, for example, the reaction of a lower oxidation state of metal ion with peroxyl radicals, etc. Ions of Al, Ti, Zn and V usually reduce the rate of oxidation. 

Although the redox reaction mechanisms of iridium oxide are still not clear, most researchers believe that the proton exchange associated with oxidation states of metal oxides is one of the possible pH sensing mechanisms [41, 87, 100, 105], During electrochemical reactions, oxidation state changes in the hydrated iridium oxide layer are... 

X-ray photoelectron (XPS-ESCA) spectroscopy uses inner-shell electron transitions to determine the oxidation state of metals in bioinorganic species. 

A type of radiation that was not available earlier came into existence and eventually became available to soil scientists. This is the radiation given off by synchrotrons that emit what is called synchrotron radiation (originally considered a waste product of acceleration electrons close to the speed of light). It is described as similar to bright X-rays. This electromagnetic radiation has been used to successfully elucidate the structure and oxidation states of metals in soil and thus their likelihood of becoming environmental pollutants [34],... 

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

Hydroxylamine is used as a reducing agent in many inorganic and organic synthetic reactions. Other applications of this compound include purification of aldehydes and ketones dehairing of hides as an antioxidant for fatty acids to stabilize lower oxidation states of metal ions for analysis and in photography. 

In the radiolysis of nitric acid, HNO2 and H2O2 are formed. They strongly affect the solvent degradation, oxidation states of metal ions, and corrosion condition of the material. However, HNO2 and H2O2 are not coexisting because the next reaction will take place. 

Oxidation states of metals Redox titrations, XPS, EELS, AES and X-ray absorption spectroscopy... 

Traditionally, X-ray absorption edge measurements have been used to determine oxidation states of metals in complex materials. The extended X-ray absorption fine structure (EXAFS), on the other hand, provides structural information such as bond distances and coordination numbers even with powdered samples, crystalline or amorphous, the fine structure essentially resulting from short-range order around the absorbing atom. The technique is also useful for studying solid surfaces (SEXAFS). The observation of fine structure beyond the K-absorption edges of materials dates back to... 

X-ray absorption near edge structure (XANES) is useful in determining the coordination number and the oxidation state of metal ions (Sankar et al, 1983). In Figs. 2.16 and 2.17 we show the XANES of Co and Cu in some compounds as well as catalysts. The ls-3[Pg.99]

A slightly different problem arises when we consider the lower oxidation states of metals. We know that CaF2 is stable. Why not CaF as well Assuming that CaF would crystallize in the same geometry as KF and that the internuclear distance would be about the same, we can calculate a lattice energy for CaF. U0 = — 795 kJ mol"1. The terms in the Bom-Haher cycle are... 

The high oxidation states of metal ions occurring at the beginning of the transition and actinium series usually are found in complex oxycations of the types MO + and M0 +. The remarkable... 

Similar processes are observed with transition metal dithiolene complexes, in which a range of formal oxidation states of metal ions may be accommodated. In reality, this is... 

Oxidation-reduction Specific oxidation states of metals, ferrocyanide, permanganate,... 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

Little work has been conducted on the oxidation state of metals within atmospheric samples due to the lack of techniques which work at very low metal... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

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