Uranyl compound

Hexavalent uranium is far more easily reduced than either hexavalent molybdenum or tungsten the potential for the UO — U+4 couple is close to that for the Cu2+ — Cu couple. 

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Zachariasen, W. H. (1954). Crystal chemical studies of the 5/-series of elements. XXIll. On the crystal chemistry of uranyl compounds and of related compounds. Acta Crystallogr. 7, 795-799. 

The general electrochemical behavior of uranium in aqueous solutions is dominated by the reduction of the hexava-lent uranyl moiety, 1102. As shown in Table 1, the potential for the UO2 /UO2 couple is 0.089 0.002 V versus SHE, as determined from formal potential data in C104 solutions (0.5-3.0 M) [49, 50aj. The electrochemical reduction of uranyl compounds has been a thoroughly studied... 

Burns, P. C., Deely, K. M., Skanthakumar, S. 2004. Neptunium incorporation into uranyl compounds that form as alteration products of spent nuclear fuel Implications for geologic repository performance. Radiochim. Acta, 92, 151-159. 

E. Rabinowitch and R. L. Belford, Spectroscopy and Photochemistry of Uranyl Compounds , Pergamon, Oxford,... 

The closed-shell system thorium(IV) shows no electron transfer satellites (15). Protactinium is too rare and too radioactive to be measured by us, but certain uranyl compounds such as RbUC>2(02N0)3 show satellites about 3.5 eV above /(IJ4/5/2) and 7(U4/7/2) probably due to electron transfer to the empty 5/ shell. The somewhat peculiar photo-electron spectra of uranium compounds are further discussed Chapter IIF. 

Rabinowitch, E. Bel ford, R. L. "Spectroscopy and Photochemistry of Uranyl Compounds" Pergamon, Oxford, 1964. 

The ground state of the uranyl ion has a closed-shell electron configuration. There is a characteristic absorption 25 000 cm (400 nm) which frequently gives uranyl compounds a yellow colour (though other colours like orange and red are not infrequent). This absorption band often exhibits fine structure due to progressions in symmetric 0=U=0 vibrations in the excited state, sometimes very well resolved, sometimes not (Figures 12.1 and 12.2). 

Rare organoimides U(MeCp)3(=NR) have been made by oxidation of U(MeCp)3(thf) with RN3 (R = MesSi and Ph). The U-N bond length of 2.109 A is regarded as having multiple-bond character (cf. the U=0 bond in uranyl compounds). 

Tripathy, Sahoo, and Patnaik have reduced alcoholic solutions of uranyl compounds photochemically, but the photolytic reduction is not complete in a reasonable time, and the products are partially hydrolyzed and/or hydrated. A large amount of acetic anhydride is required for complete conversion of the oxide acetate into the desired product and subsequent recovery. Oxide oxalate and oxalate can, however, be prepared by the addition of oxalic acid to the reduced aqueous solutions. The method reported below is unique in the sense that reduction is complete in a fairly short time and the yield is almost quantitative. Two alternative procedures are outlined. 

The uranous compounds correspond to the basic oxide, UOj, and are usually green or blue in colour. They may be prepared by reduction of uranyl salts in solution under the influence of light (see below), but they are the more unstable and the solutions are readily oxidised back by air to uranyl compounds, especially in the presence of platinum black or of salts of iron or copper. In the uranous compounds uranium shows considerable chemical similarity to thorium, the terminal member of Group IV (see p. 4). 

Uranous fluoride is a green amorphous powder, insoluble in water, hardly attacked by dilute acids and-only dissolved with difficulty by concentrated acids, except in the case of nitric acid which readily decomposes it. When heated in the air it loses fluorine and leaves a residue of urano-uranic oxide. At ordinary temperatures it is gradually oxidised to the uranyl compound, UOjFj. If heated in absence of air it melts at about 1000° C. ... 

More recently Carnall, et al. (6) have pointed out that the symmetric stretching frequency cannot be determined in many uranyl compounds, thus preventing the calculation of Fro- Since the error introduced by neglecting bond-bond interactions should be only several percent, they suggest use of the asymmetric stretching frequency only, thus permitting modification of the equation to... 

Table 12.4 lists the signatures for processes using UF gas. In the conversion step, one could look for uranyl fluoride, but the isotopic ratio would not change from uranium ore (i.e., no fractionation). However, an analytical method that yielded oxidation states of atomic uranium and fluoride, such as ESCA (electron spectrometry for chemical analysis), could be used to identify the uranyl compound. 

Nondestructive high Z for spot sizes of 0.1 mm (i.e. boron nitride, uranyl compounds)... 

Analogous and carbonates can be assumed to be isostructural with the well-characterized uranyl compounds, albeit with slightly shorter bond distances to reflect the actinide contraction. For the triscarbonato complexes of Np this has been confirmed by the structures of K4Np02(C03)3 by single-crystal structure determinationof [(CH3)4N]Np02(C03)3. " ... 

Bums et al. [2] developed a detailed stmctural hierarchy for inorganic uranyl compounds, and included both minerals and synthetic phases. At that time, 180 stmctures were available for inclusion. Bums [22] expanded the stmctural hierarchy and updated it in the case of minerals only. Bums [3] further developed the entire hierarchy and included coverage of 368 mineral and S5mthetic phases. The stmctural hierarchy is based upon the linkages of those polyhedra that contain higher-valence cations. In every case this includes polyhedra with U , and often it includes one or more other t5q>es of cation polyhedra. For the purposes of the hierarchy, bonds to low-valence cations and H bonds are ignored (although these bonds are important for the stability of the entire stmcture). 

Since first data concerning structures of uranyl compounds with CV(U(V1)) = 5 appeared in 1999, it is of great interest to include these structures into the analysis of the characteristics of the U(V1) VD polyhedra. Initial crystal structure information was extracted from the known stmcture databases [2, 3] and analyzed using the TOPOS software [16]. Stmctures were included into consideration under conditions that crystallographic agreement index (7 i) is less than 0.10 and that there is no disorder in the U or O sites in the UO polyhedra. In total, 908 compounds were considered that contained 1465 crystallographically independent U(V1) atoms. From these atoms, 15, 306, 906, and 238 have coordination numbers of 5, 6, 7, and 8, respectively. 

Table 3. Electron-donor characteristics of some ligands in the structures of uranyl compounds...

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