Cationic complexes, trivalent uranium

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 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. 

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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