Dimer actinide

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

Is it possible that other actinides can also form dimers We already mentioned that Th2 has been detected in the gas phase and in a rare gas matrix. We have studied this dimer and the dimers of Ac and Pa.95 Some major findings are reported here. 

With the Pa2 dimer, we have reached the maximum bonding power among the actinide dimers. In U2 the bond energy decreases and the bond length increases, which is from the increased stabilization of the 5f orbitals and the corresponding destabilization of 6d. Large transfer of electrons from... 

In an investigation of the extraction of trivalent actinides, An(III) from 0.01 M nitric acid solutions of various LiNOj concentrations into o-xylene containing the tertiary amine salt trilaurylmethylammonium nitrate, TLMA HNOj, Van Ooyen [29] found that the amine was monomeric only at very low concentrations (<0.1M in the organic phase) but at higher concentration formed both dimers and trimers. 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

As an example of actinide dimeric complexes, the series of compounds [M(0-2,6-z-Pr2C6H3)3]2 970 (M = La, Nd, Sm, Er, U), sterically demanded by z-propyl substituents, were isolated [308,310a] they are held both in the solid state and in solution by 7i-arene interactions ... 

In Harrod s original report of the screening of metallocenes as dehydrocoupling catalysts, Cp MMe2 (M = U and Th) was studied and although these actinides did promote the reaction of PhSiH3, only dimers or small oligomers were produced.6b... 

Hexavalent. As with most reactions, the hydrolysis of U02 + is the best studied of the hexavalent actinides. The hydrolysis of U02 + begins at pH 3, while the onset for the hydrolysis of Np02 + and Pu02 + each occur at a higher pH. The monomeric hydrolysis products of the uranyl ion, U02(0H) n = 1, 2) can be studied in solutions with uranimn concentrations less than 10 M. For solutions with higher uranium concentrations, multinuclear cationic species dominate the speciation, for example, (U02)2(0H)22+, (U02)3(0H)42+, and (U02)3(OH)s+. These cations have been crystallized from solutions with the formulas (U02)2(at2-OH)2(OH2)6 + and (U02)3(M3-0)(/x2-0H)3(0H2)6+ (21). For Np and Pu, the dimer of the first hydrolysis product, (An02)2(OH)2 + (22), has also been identified but not fully stracturally characterized. 

The best-studied aqueous actinide halide systems are the fluorides and the chlorides. Fluoride and chloride ions are added to the actinyl centers in a stepwise fashion (Scheme 4). The end member in the actinyl fluoride system is the pentafluoride, An02F5 . For the uranyl analog, the U = O and U-F bond distances were found to be 1.79 and 2.18A respectively. The uranium tetrafluoride ion, U02F4 , has been isolated as a dimer with two bridging fluoride ligands. The U = O, U-Fterminai, uud U-Fbridging distauces for this complex were found to be 1.79, 2.15 to 2.20 A, and 2.30 A, respectively. 

Particular areas of interest are the chemistry early-late metal-metal bonded complexes, main group metal-transition metal dimers, transition metal carborane derivatives, and lanthanide and actinide transition metal complexes. ... 

The analogous uranium(III) compounds also show unusual reactivity patterns. For example, addition of COT to (Cp )3U yields a mixed metallocene dimer (18) bridged by a COT ligand in this reaction, the (Cp )3U complex has effectively acted as a formal three-electron reductant (equation 3)." A variety of substituted (Cp )3U complexes form adducts with CO and CNR (isocyanides) - these are rare examples of actinide metals with r-acidic ligands." Paramagnetic uranium alkyl complexes are active in a range of catalytic processes." ... 

Hydroxides. Pure and mixed metal actinide hydroxides have been studied for their potential utility in nuclear fuel processing. At the other end of the nuclear cycle, the hydroxides are important in spent fuel aging and dissolution, and environmental contamination. Tetravalent actinides hydrolyze readily, with Th more resistant and Pu more likely to undergo hydrolysis than and Np. All of these ions hydrolyze in a stepwise marmer to yield monomeric products of formula An(OH) with = 1,2,3 and 4, in addition to a number of polymeric species. The most prevalent and well characterized are the mono- and tetra-hydroxides, An(OH) and An(OH)4. Characterization of isolated bis and tri-hydroxides is frustrated by the propensity of hydroxide to bridge actinide centers to yield polymers. For example, for thorium, other hydroxides include the dimers. 

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