Metals lower valent

The purple permanganate ion [14333-13-2], MnOu can be obtained from lower valent manganese compounds by a wide variety of reactions, eg, from manganese metal by anodic oxidation from Mn(II) solution by oxidants such as o2one, periodate, bismuthate, and persulfate (using Ag" as catalyst), lead peroxide in acid, or chlorine in base or from MnO by disproportionation, or chemical or electrochemical oxidation. 

Conversely, if the pressure of O2 above a crystalline oxide is decreased below the equilibrium value appropriate for the stoichiometric composition, oxygen boils out of the lattice leaving supernumerary metal atoms or lower-valent ions in interstitial positions, e.g. ... 

An explanation that may be suggested of these facts is that solid solutions of a quadrivalent metal (zinc) in a tervalent metal (aluminium) tend to be unstable because of the difficulty of saturating the valency of isolated quadrivalent atoms by bonds to its lower-valent ligates. With zinc as the solute an increase in free energy at the lower temperatures would accompany the separation into the zinc-poor a phase, in which the versatile zinc atoms tend to assume the valency 3 (less stable, however, for them than their normal valency) in order to fit into the aluminium structure, and the zinc-rich a phase, in which the concentration of zinc atoms is great enough to permit the extra valency of zinc to be satisfied through the formation of Zn-Zn bonds. 

Restoration of the decayed species to its active valence is thus the key to reactivating the catalyst. It has been known that organic halides with activated C—Cl bonds can add to lower-valent transition metals and convert them to their higher oxidation states by oxidative addition (12-16) ... 

There are several bottom-up methods for the preparation of nanoparticles and also colloidal nanometals. Amongst these, the salt-reduction method is one of the most powerful in obtaining monodisperse colloidal particles. Electrochemical methods, which gained prominence recently after the days of Faraday, are not used to prepare colloidal nanoparticles on a large scale [26, 46], The decomposition of lower valent transitional metal complexes is gaining momentum in recent years for the production of uniform particle size nanoparticles in multigram amounts [47,48],... 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

Additionally, the ability of H2O2 to oxidize a metal to a higher-valent state, resulting in a more insoluble hydroxide (higher-valent metal hydroxides are more insoluble at a particular pH than the hydroxide of the metal in a lower-valent state) has been pointed out in this study. 

Phosphorus trifluoride is a ligand that is used extensively in coordination chemistry. It substitutes readily into various metal carbonyl complexes using either thermal or photochemical techniques. As a ligand, it is unique in its similarity to carbon monoxide in lower-valent organometallic compounds. In its role as a model for CO, a number of studies are possible that cannot be done on the carbonyls themselves.1 The name normally used for PF3 in complexes is trifluorophosphine. 

The alcoholysis of a lower valent metal dialkylamide can also lead to oxidation of the metal, typically with evolution of hydrogen (equations 13 and 14).74,81... 

Catalyzed oxidations.1 In catalytic procedures with Ru04, periodate or hypochlorite are generally used as the stoichiometric oxidants. The addition of acetonitrile, which is inert to oxidation but an effective ligand for lower valent transition metals, results in much higher yields. A third solvent, chloroform, also plays a significant part. The ruthenium tetroxide is generated in situ from RuCl, (H20)n or Ru02 with sodium or potassium metaperiodate sodium hypochlorite is less effective. 

Fig. 18. Schematic diagram for a binary alloy with a passivating oxide film in contact to electrolyte with the reactions of (1) oxide formation, (2) electron transfer, and (3) corrosion, including (4) oxidation of lower-valent cations and the indication of ionic and atomic fractions X as variables for the composition of the layer and the metals surface.

The analysis of several pure metals and binary alloys yields generally at least a duplex and in some cases a multilayer structure of the passive film, as depicted schematically in Fig. 19. These systems have been examined with surface analytical methods, mainly XPS, but also ISS in some cases. The systematic variation of the electrochemical preparation parameters gives insight to the related changes of layer composition and layer development, and support a reliable interpretation of the results. Usually the lower valent species are found in the inner part and the higher valent species in the outer part of the passive layer. It is a consequence of the applied potential which of the species is dominating. Higher valent species are formed at sufficiently positive potentials only and may suppress the contribution of the lower... 

Electrochemical procedures can thus be used in the production of solid catalysts for the reduction of higher valent metal ions, usually present as oxyanions, to a lower valent state, where they are less acidic. Also, the above deposition-precipitation method can be extremely well controlled by electrochemical means. 

By doping a primary catalyst component with lower-valent metal cations, additional oxygen vacancies will be created which facilitate the incorporation of electrophilic oxygen species chemisorbed on the surface into the bulk where they will not oxidize adsorbed methyl radicals. Also, the promoter oxide should be basic, not be reducible, oxidizablc, or easily volatiz-ablc. It should form a mixed oxide with the main component which may be possible if the ionic radii arc similar. According to these rules, the expert system proposes as potential catalyst components combinations of substances with appropriate chemical and physico-chemical properties (Table 2). Many of these systems already have been described in the literature... 

Reaction between a reducing alkyl metal compound and a transition metal compound proceeds through a series of alkylated transition metal compounds which can decompose into lower valent transition metal compounds and free radicals (284, 289, 318—332). 

Passivation potential — A metal turns passive if the electrode potential is shifted above the passivation potential Ep into the passive range of the -> polarization curve (Fig. 1). This critical potential depends on the thermodynamic properties of the metal. In many cases it equals the value deduced from the thermodynamic data for the formation of an oxide layer of the metal in aqueous electrolytes according to Eq. (1). This reaction is - pH dependent by -0.059 V/pH. In some cases it corresponds to the oxidation of a lower valent to a higher valent oxide (Eq. (2)). For iron the passivation potential in acidic electrolytes has been explained by Eq. (3). 

The cyanogen molecule, N=C—C=N, is linear. It dissociates into CN radicals, and, like RX and X2 compounds, it can oxidatively add to lower-valent metal atoms giving dicyano complexes, for example,... 

The first key element in the uptake of these two metal ions is that the substrate is the lower valent state species, Cu(I) in the case of copper and Fe(II) in the case of iron (Dancis et al., 1990, 1992 Hassett and Kosman, 1995 Kosman, 1993). Normally, these reduced valence species are provided by the action of plasma membrane metal reductases, an activity in yeast provided predominantly by the product of the FREl gene (Dancis et al., 1992). However, Fe(II) [or Cu(I)] provided exogenously to the cell is equally competent for uptake and, in most experimental regimes, is added directly or generated in situ by the addition of a strong reductant like ascorbate or dithionite. Cu(I) is the direct substrate for uptake, through the Ctrlp copper permease in most yeast strains (Dancis et al., 1994). However, the presence of Fe(II), although required, alone is not... 

The rate constants and the associated free-energy snrfaces available to the peroxide and native intermediates deserve comment since they differ overall by nearly 10 (or ca. Vkcalmol in absolnte valne). Given the relatively electronentral nature of electron transfers between the copper sites (the E° values for the three sites differ overall by only 60 mV), the differences in rate in the first instance reflect the difference in the E° value for le versus 2e reduction of dioxygen (leading to the peroxy intermediate) and peroxide (leading to the native intermediate). Second, the differences reflect the work available from the favorable 4e reduction that drives the turnover from native intermediate to fully reduced enzyme primed, now, to react with O2. This latter process, k 100 s (compare to k = 0.34s for decay of the native intermediate to fully oxidized enzyme), is functionally equivalent to the reductive release of Fe + from Fe +-transferrin catalyzed by the membrane metalloreductase, Dcytb in both cases, the lower valent metal species is more loosely coordinating. Whereas Fe + dissociates in the latter case, in MCO turnover the bound water(s) dissociate. 

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