Valent states

Qiemical shift from zero-valent state... 

The data obtained up to the present time show that the method of catalyst preparation by the reaction of organometallic compounds with surface reactive groups may be applied to generate both isolated ions of transition metals (in various valent states) or superfine metal particles on the surface of the support. 

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

Table 2.4. Classification of acceptor atoms in their normal valent states (Ahrland, Chatt Davies, 1958)... 

Chatt, J. (1958). The stabilisation of low valent states of the transition metals. Journal of Inorganic Nuclear Chemistry, 8, 515-31. 

The synthesis of metal nanoparticles via the controlled decomposition of pre-prepared organometallic complexes or metal carbonyls where the metals are already in the zero valent or low-valent state has been known since 1970. The first examples were Pd- and Pt-dibenzylideneacetone complexes where the coordinated ligands detached using either hydrogen of carbon monoxide under mild conditions to give the respective metal nanoparticles [310]. 

The chemical state of the metal can play a decisive role on the reaction mechanism. In TWC, Rh is thought to remain in the zero-valent state, which favors NO dissociation [77,78], However, the role of the OSC materials is complex, and it is not inert with respect to NO activation. Ranga Rao et al. [79] showed that, when bulk oxygen vacancies are formed in a reduced Ce06Zr04O2 solid solution, NO was efficiently decomposed on the support to give N20 and N2. Further studies by the same group... 

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

If the reaction in which the metallic fraction serves as a catalyst produces water as a by-product, it may well be that the catalyst converts back to an oxide. One should always be aware that in fundamental catalytic studies, where reactions are usually carried out under differential conditions (i.e. low conversions) the catalyst may be more reduced than is the case under industrial conditions. An example is the behavior of iron in the Fischer-Tropsch reaction, where the industrial iron catalyst at work contains substantial fractions of Fe304, while fundamental studies report that iron is entirely carbidic and in the zero-valent state when the reaction is run at low conversions [6],... 

Reductive eliminations can be promoted by stabilisation of the low-valent state of the product. This means ligands that are good K-acceptors, bulky ligands, and ligands preferring bite angles more suited for tetrahedral than for square-planar complexes, when we deal with group 10 metals. 

To avoid coextraction of iron, the iron is initially reduced to its Il-valent state. Then zinc is extracted as a zinc chloride complex into an organic solution containing tributylphosphate (TBP). Zinc is stripped from the organic solution with water or dilute hydrochloric acid (Fig. 14.5). The resulting strip solution is evaporated, either (1) after addition of sulfuric acid, giving a dilute hydrochloric acid condensate and a zinc sulfate precipitate, or (2) directly without any addition, giving a dilute hydrochloric acid condensate and a concentrate zinc chloride product solution. 

It may be mentioned that starting with ash and soot from crude oil-fired stations, the resulting metals are the same, but the main leaching residue is carbon. This residue is initially burned and the ash is leached again to increase the total yield of vanadium. In the same operation, the concentration of iron is reduced by precipitation of jarosite. During leaching, the redox potential is controlled by SO2 addition to keep vanadium in its IV-valent state. 

Methylococcus capsulatus, diferric iron cluster, 43 362-363 mixed-valent state, 43 389 s derived from reaction mechanism, 43 391-393... 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

Commercial separations involving the oxidation to the tetra-valent state are limited to the removal of cerium after oxidation. These separations, which are based upon reduced basicity in the tetravalent state, include ... 

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