Oxidation trivalent uranium

Oxidation of the sterically crowded complex Cp 3U provided access to [Cp 2U]2(/i-0) as the first molecular trivalent uranium oxide. The U-O-U angle in this molecule is 171.5(6)°.85 Reduction of the uranium(iv) thiolates... 

Another most interesting and unusual result has been reported regarding the formation of an U(III) oxide complex, [(Cp )2U]2( x-0) (Fig. 5) obtained by the (r] -C5Me5)3U reduction system (Scheme 4) [107]. This has been claimed to be the first molecular trivalent uranium oxide so far reported. The complex was isolated from a reaction of (r -C5Me5)3U with KCg in toluene. Probably this is the first example of an SIR process in which the CsMes" reduction precedes the U(III) electron transfer. 

Uranium exists in the +3, +4, +5, and +6 oxidation states. The +5 state dispropor-tionates to the +4 and +6 states and is of little importance. Trivalent uranium reduces water and therefore there is no stable aqueous chemistry of U3+ although compounds do exist. 

In conclusion, this chapter has covered the most recent literature of trivalent uranium, an oxidation state that only recently came increasingly under the spotlight and that currently can be regarded as a rapidly rising star in the held of... 

Simple compounds of trivalent uranium have a long history thus, in this review the emphasis is on the developments since the Gmelin Uranium (i) supplementary volumes (la-j). However, reference is made to earlier work if it facilitates understanding and avoids a disjointed account. A compact account of most of the earlier work can be found in the second edition of The Chemistry of the Actinide Elements (2). Solid compounds, for which oxidation state concepts are inappropriate, have usually been excluded. 

Trivalent uranium ion reduces water to hydrogen. Hence, stable aqueous solutions of trivalent uranium compounds cannot be prepared. Compounds of tetravalent uranium are generally similar to those of zirconium or thorium, except that some uranium compounds can be oxidized to the hexavalent form. Compounds of pentavalent uranium are of little importance because they disproportionate readily into tetravalent and hexavalent forms. The properties of hexavalent uranium are generally similar to those of hexavalent molybdenum or tungsten. In aqueous solution hexavalent uranium forms the uranyl ion UO2 ... 

While actinide metals are easily soluble in some mineral acids such as HCl an.d HCIO4, they are typically found in solution in valences IV to VI. However, Baluka and co-workers first reported in 1981 that dissolution of U and Np metals in superacids produces initially trivalent actinides (Baluka et al. 1981). The UV-visible spectra of the U and Np solutions, shown in fig. 2, are characteristic of the trivalent actinides found in aqueous acidic or basic systems. Avens and colleagues (1988) have dissolved both Pu and Am metal and observed trivalent species by UV-visible spectroscopy. The stability of oxidation state (111) for U, Np and Pu is all the more striking especially in light of the lower stability of the series U (lll)-Pu (111) relative to higher oxidation states e.g., trivalent uranium (111) is able to reduce water under normal conditions. In addition to the unusually low oxidation state obtained, another feature of actinide dissolution in... 

U(Tp )2CH2Ph] and the high thermodynamic stabihty of the +4 oxidation state.The trivalent uranium complex [U(Tp )2(CH2Ph)] has been investigated by DFT as catalyst to promote the fimctionahzation ofC02 and CS2-It has been found that the interaction between CO2 and CS2 is not with central uranium atom, but due to the steric hindrance the first step is the cleavage of the U—C benzyl bond to form a C—C covalent bond. " ... 

Andersen, Inorganic Chemistry 1986, 25, 1756. (c) Chemistry of trivalent uranium metallocenes electron-transfer reactions. Synthesis and characterization of [(MeC5H4)3U]2E (E = S, Se, Te) and the erystal structures of hexakis(methylcyclopentadienyl)sulfidodiuranium and tris(methylcyclopentadienyl)(triphenylphosphine oxide)uranium, J. G. Brennan, R. A. Andersen, Inorganic Chemistry 1986, 25, 1761. 

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

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