Uranium IV and VI

The reactions of the photochemically prepared [U02] ion with a series of chromium, ruthenium, and cobalt complexes have been studied/ An application of the Marcus equation to the cross-reaction data yields a [U02] self-exchange rate constant in the range of 1-15 s The reactions with the chromium and 

The reductions of a series of mixed polypyridine/oxalate complexes of Cr(III), [CrL3 (ox) ] (n = 1-3), by pulse radiolytically generated Zn(I) and Cd(I) proceed with rate constants in the range of 1.6-2.5 x 10 Ms . The reduction of [Co(ox)3] by Cu, generated by the 8203 reduction of Cu, occurs by an outer-sphere mechanism and is inhibited by added edta or oxalate ions.  

The rate constants for the oxidation of Ni(II) and Cu(II) tetraaza macrocycles ([14]aneN4 and Me6[14]4,ll-dieneN4]) by the Ag ion in perchlorate media are independent of A very small Ag self-exchange rate constant of 2 x 

10 M s derived from the cross-reaction with [Ni([14]aneN4)], suggests a possible inner-sphere contribution. The kinetic data for the noncomplimetary oxidations of a series of Cr(III) tetraaza macrocycles by both [Ce(OH)] and [Ce(OH)2] in acidic sulfate media indicate that the one-electron oxidation of Cr(IV) to Cr(V) is the rate-determining step.  

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We have previously determined the enthalpies of formation of several perovskite +4 and +6 oxides (3, ). Our obj ective in this study was to determine the enthalpy of formation of BaU03 and to evaluate the relative stability of uranium(IV) and (VI) in comparable complex oxides, especially in comparison with binary oxides, halides, and aqueous ions. 

Benzopurpln preelpltateia uranium (IV) and (VI) Benzoylaoetone precipitates uranium (VI). 216 5-Bromo-7-carboxy-8-hydroxyQulnollne precipitates uranium (VI), copper, zinc, cadmium, mercury, and lead.- 5-Carbo3ty-8-hydro tyaulflollne precipitates uranium (VI)... 

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

A product of composition UF5-2SC(NH2)2 has been obtained by treating UF6 in methyl cyanide with thiourea this may be a mixture of uranium(IV) and uranium(VI) species. 

The (IV) and (VI) are the important oxidation states and therefore tlie more important phases of the chemistry of uranium may be related to the two oxides UO> and UO3. uranium dioxide and uranium trioxide. A series of salts such as the chloride and sulfate, UC14 and U(S04)2 9H20 is... 

Practically all the uranium alkoxides, excluding the polymeric derivatives of uranium (IV) and those of dioxouranium (VI), are rather highly soluble in both polar and nonpolar organic solvents (Table 12.20). Lowvalent ( IT-HI) derivatives are not volatile because of redox transformations that apparently accompany the heating. The volatility of alkoxoderivatives increases from tetrav-alent to hexavalent ones and in each homologous series with the increase in the size and ramification of the radical. 

Uranium(VI) (uranyl) and iron(III) salts interfere and yield brown and deep-green colourations respectively these colours are destroyed by the addition of tin(II) chloride, for uranium(IV) and iron(II) salts do not react with chromotropic acid. Mercury salts give a yellow and silver salts a black stain on drop-reaction paper the colour due to titanium is, however, still perceptible. 

Extraction of Actinides(IV) and (VI). Table VI shows the D s for representative hexa- and tetravalent actinides from 3 M HNO3 at 50 C for DHDECMP, HHDECMP, and 0( )D[IB]CMPO. The same concentration of extractants in DEB were chosen as used to measure the fission product D s. The D for Np(V) was found to be 0.64 from 3 M HNO3 at 50°C using 0.5 M 0(j)D[IB]CMP0 in DEB. The data for U (VI), Np(IV), and Pu(IV) for DHDECMP are in general agreement but somewhat higher than the values reported by Schulz and Mclsaac (1). This disparity is probably due primarily to the lower purity of the CMP extractant used in the earlier work. The trends in distribution ratios for tetravalent actinides and uranium follow the same trends as D for the three classes of CMP extractants. 

Alkoxide complexes. In contrast to the propensity of many uranium(V) species to disproportionate to uranium(IV) and uranium(VI), homoleptic uranium(V) alkoxide compounds are quite stable toward disproportionation. Gilman and co-workers reported the synthesis of dark brown uranium(V) pentakis(ethoxide) from a metathesis reaction between UCI4 and four equivalents of sodium ethoxide. . In this early report, it was noted that better yields were obtained when no great care was taken to exclude air from the reaction, and in the presence of oxygen, the product yield was 80%. The mechanism shown in Equations (36) and (37) was suggested for this reaction. 

The sucessful experiments for the retention of plutonium onto alumina from TTN0 -HF solution gave enough confidence to recomend the proposed method to separate traces of plutonium from waste solutions in the presence of macroamounts of uranium (VI). Of course, only macroamounts of thorium, uranium (IV) and rare earths are serious interfering ions, since they precipitate with HF. The behavior expected for neptunium in the same system should be similar to plutonium, thorium and rare earths. The retention of neptunium from HNO - HF solutions is in progress. The sorption yield for Pu was around 95%. The sorption mechanism is not well established. Figure 3 shows the proposed flowsheet for recovery of Pu traces from reprocessing waste solutions. 

The donor action of [NOs]" as an anionic ligand towards thorium(iv) and uranium(iv) in the presence of trimethylphosphine and tris(dimethylamino)phosphine oxide in aqueous media has been found to be very similar. The larger nitrate ion was observed to form more stable species with thorium than the chloride ion whereas in the uranium(iv) case both complexes formed equally readily. A study of the sulphate complexes of uranium(iv), neptunium(vi) and plutonium(vi) in HCIO4-H2SO4 solution showed the stability constants to follow the order U[Pg.453]

CC14 and CHC13, and is quite soluble in C6H6, CS2, and SOCl2-Disproportionation into uranium(IV) and uranium(VI) occurs rapidly in dimethyl sulfoxide as well as in water, alcohols, and amines. It melts at 147-149° in a sealed tube. 

Baston et al. [60] studied the samples of ionic liquid after the anodization of uranium metal in [EMIMjCl using the U Lm-edge EXAFS to establish both the oxidation state and the speciation of uranium in the ionic liquid. This was part of an ongoing study to replace high-temperature melts, such as LiQ KQ [61], with ionic liquids. Although it was expected that, when anodized, the uranium would be in the +3 oxidation state, electrochemistry showed that the uranium is actually in a mixture of oxidation states. The EXAFS of the solution showed an edge jump at 17166.6 eV, indicating a mixture of uranium(IV) and uranium(VI). The EXAFS data and pseudo-radial distribution functions for the anodized uranium in [EMIMjCl are shown in Eig. 4.1-12. 

Thorium, neptunium and plutonium are usually (although not exclusively) extracted or adsorbed onto resins in the tetravalent oxidation state while uranium (VI) is the most common oxidation state in separations. In most separations of the actinides, there exists a great difference in the extractability of the (IV) and (VI) oxidation states relative to the trivalent oxidation state. It is reasonable to expect, therefore, that the removal of the lanthanides and transplutonium actinides from the light actinides is readily accomplished. In fact, the low extractability of the trivalent state, and the ease of reduction of plutonium forms the basis for the isolation of Pu from the other... 

Kumar, N. and Tuck, D.G. (1984) The direct electrochemical synthesis of neutral and anionic halogeno complexes of uranium(IV) and uranium(VI) , Inorg. Chem. Acta, 95,211-215. 

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