Trivalent uranium cationic

The heteroscorpionate pyrazolylborate derivative K[H2B(pzf Bu,Me)(pzMe2)] upon reaction with UI3(thf)4, afforded a series of seven-coordinated trivalent complexes of formula U /t, -BpzM " Me (Hpzf"Bu Me)I(p-I)]2 (55). The absence of empty coordination sites provide these complexes with a remarkable stability. Along the same line, the reaction of bis(2-mercapto-2-methylimidazolyl)borates [H(R)B(tim)2 R = H, Ph] with UI3(thf)4 in the presence of Tl(BPh4) in THF gave another rare case of a cationic trivalent uranium complex U[H(R)B-(tim)2]2 (thf)3 (BPh4) (56). 

In this P31c structural type, the two cationic sites are always fully occupied by M and M , respectively, giving systematically a VEC = 16. These compounds exist in the Nb-Cl, Nb-Br, Ta-Cl, Ta-Br systems with all of the trivalent rare-earth ions and trivalent uranium but only cesium (occasionally rubidium) as the monovalent countercation. Very recent work on the Ta-Cl system has, however, realized the possibility of replacing the trivalent rare earth ion by divalent cations, giving the CsM Ta6Cli8 series with = Ba, Pb, Sr, Eu, This new series, for which... 

The separation of the lanthanides from thorium, uranium, plutonium, and neptunium can fairly readily be achieved by exploiting the greater extractability of the higher oxidation states of the light-actinide elements. However, the transplutonium actinides do not have stable higher oxidation states. In this case, separation of the lanthanide fission products from the transplutonium actinides must exploit the small differences in the solution chemistry of the lanthanides and actinides in the trivalent oxidation state. It is the separation of the lanthanides from the trivalent actinide cations that is the focus of this chapter. 

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 activity was transported to ARCA II with a He(KCl) gas-jet within about 3 s. After deposition on a titanium slider it was dissolved and washed through the 1.6x8 mm column (filled with the cation-exchange resin Aminex A6, 17.5 2 pm) a flow rate of 1 mL/min with 0.1 M HNOj/5-10 4 M HF. 85% of the W elute within 10 s. Neither divalent or trivalent metal ions nor group-4 ions are eluted within the first 15 s. Also the pseudo-homologue uranium, in the form of U022+, is completely retained on the column. 

In the case of pentavalent cations, the conditional interaction constant (Table I) obtained for Np02" (log P = 4.6) shows a relatively low affinity of the neptunyl cation with the humic acids as it could be predicted from the charge of the ion. The interaction constant obtained for the hexavalent cation (U) with humic acid (Table I) is independent of pH (4-5) in the non-hydrolysis pH-range but some variation with uranium concentration is observed as for trivalent cations. Moreover, the complexation of uranium to humic substances is of the same order of magnitude than the complexation of trivalent actinides which corroborates chemical analogy between both cations. 

From the above discussion it follows that tetravalent and hexavalent thorium, uranium, and plutonium can be separated from the trivalent rare-earth fission products by taking advantage of differences in complexing properties. More highly charged cation fission products, such as tetravalent cerium and the fifth-period transition elements zirconium, niobium, molybdenum, technetium, and ruthenium, complex more easily than the trivalent rare-earths and are more difficult to separate from uranium and plutonium by processes involving complex formation. 

The most prevalent form of uranium in aqueous solution is the tight yellow, fluorescent uranyl ion U02. The U cation (green in solution) can be obtained by strong reduction of U(V1), but readily oxidizes back to U02 in air. The pentavalent ion U02 can be reversibly formed by reduction of U02, but it readily disproportionates into U(IV) and U(VI). The trivalent U can be formed by reduction of U(IV) but is unstable to oxidation in aqueous solution. 

In the assessment of the refining performance of uranium, systematic data has been reported for the chemical properties of uranium complex in various alkali chlorides such as LiCl-RbCl and LiCl-CsCl mixtures [3-5], Information on the coordination circumstance of solute ions is also important since it should be correlated with stability. The polarizing power of electrolyte cations controls the local structure around neodymium trivalent Nd " " as an example of f-elements and the degree of its distortion from octahedral symmetry is correlated with thermodynamic properties of NdClg " complex in molten alkali chlorides [6]. On the other hand, when F coexists with Cr in melts, it is well-known that the coordination circumstances of solute ions are drastically changed because of the formation of fluoro-complexes [7-9]. A small amount of F stabilizes the higher oxidation states of titanium and induces a negative shift in the standard potentials of the Ti(IV)ITi(ni) and Ti(III)ITi(II) couples [7, 8], The shift in redox potentials sometimes causes specific electrochemical behavior, for example, the addition of F to the LiCl-KCl eutectic leads to the disproportionation of americium Am into Am " and Am metal [9],... 

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