Tetravalent oxides

Sulfation Roasting. Acid roasting technology (Fig. 2) rehes on differences in the volatiUty of the tetravalent oxides of selenium and tellurium at roasting temperatures of 500—600°C to selectively volatilise selenium from slimes. Acid roasting uses sulfuric acid as the oxidant for the conversion of selenium/selenides and tellurium/teUurides to their respective tetravalent oxides. Typical oxidation reactions are as foUow ... 

Thorium has a wide distribution in nature and is present as a tetravalent oxide in a large number of minerals in minor or trace amounts. Thorium is significantly more common in nature than uranium, having an average content in the earth s cmst of approximately 10 ppm. By comparison, Pb is approximately 16 ppm. Thorium has a seawater concentration of <0.5 x 10 . Thorium refined from ores free of uranium would be almost... 

Tin amidinates display a rich coordination chemistry with the metal in both the di- and tetravalent oxidation states. The first results in this area were mainly obtained with N-silylated benzamidinate ligands. Typical reactions are summarized in Scheme 48. A stannylene containing unsymmetrically substituted amidinate ligands, [o-MeC6H4C(NSiMe3)(NPh)]2Sn, has been prepared accordingly and isolated in the form of colorless crystals in 75% yield. ... 

The positive charge on the transition metal in XXVII is a consequence of the tetravalent oxidation state of the transition metal in Cp2TiCl2. The active sites in traditional Ziegler-Natta polymerizations may be neutral because the transition metal is trivalent in those initiators (Secs. 8-4e, 8-4h-l). The group 3 metallocene initiators have neutral metal centers because those metals are trivalent. 

The chemical properties of germanium fall between those of silicon and tin. It forms both the divalent and tetravalent compounds, the oxidation state +4 being more stable than the +2 oxidation state. The metal is stable in air and water at ambient temperatures. However, it reacts with oxygen at elevated temperatures forming divalent and tetravalent oxides, GeO and Ge02. 

The metal also reduces the tetravalent oxide to the divalent oxide upon heating at elevated temperatures ... 

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

All the stable Si species in solutions are under the tetravalent oxidation state. No studies of electron transfer between soluble Si compounds can be found in the literature. Several investigations were devoted to the electrochemical deposition of elemental silicon from solute species. 

Isosynthesis is a potentially important variation of the Fischer-Tropsch synthesis since it yields mainly branched C4-C8 paraffins.187,188 279 280 Most of the pioneer-ing work was carried out by Pichler and coworkers. They observed that certain nonreducible tetravalent oxides (thoria, zirconia, ceria) exhibited activity in the reaction producing primarily isobutane. The optimum operating conditions... 

Cyclopentadienyl complexes of platinum are known for both the divalent and tetravalent oxidation states. Examples of both rfs and rj1 complexes are shown in equations (290) and... 

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

It is seen again that the activation energy typical of the pure metal is lowered by contact of the metal with an oxidic support. Even more important, p-type doping with bivalent oxides lowers, n-type doping with tetravalent oxides increases the activation energy relative to that observed with undoped alumina. This is consistent with the concept... 

Scheme 10). In this compound one pyrrole ring was isomerized by shifting the attachment of the chain from the a- to the (3-position GV-confused) while the formal tetravalent oxidation state is provided by the presence of the ethoxide group. The origin of the LiOEt unit is the result of a one-electron attack on thf followed by H abstraction, which accounts for the tetravalent state of uranium in the product. 

The lanthanides, unlike the transition metals and the actinides, tend not to form compounds over a range of oxidation states. The +3 oxidation state is characteristic of all of the lanthanides, and the oxide fluorides of formula LnOF (Ln = lanthanide metal) are well known. The less stable oxidation states of + 2 and + 4 are known, but the latter is represented only by the dioxides and tetrafluorides of cerium, praseodymium, and terbium, and no tetravalent oxide fluorides have been reported. 

Evidence for self-healing is not detected in identical experiments with lanthanum sealed hydrotalcite coatings. La does not have a soluble tetravalent oxide... 

Some of the rare earths exhibit two different oxidation states in the same compound. A familiar example is Tb/jOy which contains the +3 and +4 oxidation states. XPS bands due to trivalent and tetravalent oxidation states are seen in 4f, 4d and 5d bands [30]. The... 

Organoactinide chemistry initially developed around the use of the ubiquitous cyclopentadienyl ligand (see Cyclopentadienyl), Cp = C5H5, and subsequently its alkyl- and silyl-substituted derivatives. The tetravalent oxidation state... 

Cerium is the only lanthanide metal with areadily accessible tetravalent oxidation state. Synthesis and molecular structure determination of organometallic Ce(IV) complexes are of interest and a challenge, because the Ce(IV) ion is strongly oxidizing and the anionic ligands typically... 

A second reason for the wealth of chemical investigations of the early actinide elements is the relative diversity of their chemistry. While the chemistry of the later actinides is most often restricted to that of the tri- and tetravalent oxidation states, compounds of the early actinides can be isolated in all oxidation states from +3 to +7. The accessibility of a range of oxidation states is the impetus for signficant chemical interest in the early actinides, but also vastly complicates investigation of these elements under some circumstances, such as aqueous redox behavior. In the case of plutonium, ions in four different oxidation states (+3, +4, +5, and - -6) can exist simultaneously in comparable concentrations in the same solution. 

Inorganic sources include naturally occurring ores, as well as manufactured products that include oxides and metallic salts such as sulfates, chlorides, and nitrates. After grinding, the tri- and tetravalent oxides may be roasted in a kiln and reduced to divalent forms. 

Perovskite (ABO3 in which A is divalent and B is tetravalent) and pyrochlore (AaBaOi in which A is trivalent and B is tetravalent) oxide compounds have been proposed as oxygen ion conducting electrolytes for electrochemical devices. Some of the perovskite structures (e.g., BaCeO and SrCeOs) are generating interest because of... 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

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