Uranium dimer

The calculation of spectroscopic parameters of the uranium dimer, U2, received considerable attention [1135]. The bonding situation of the U2-molecule implies a large number of nearly-degenerate electronic states, which require the multi-configurational approach for the calculation of the electronic wave function. This approach was also successfully applied to [1136], to the actinide dimers Th2, Ac2 and Pa2 [1137], and to the dinuclear clusters PhU-UPh [1138], U2CI6 and U2(OCHO)6 [1139]. 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

The reddish brown pentachloride, uranium pentachloride [13470-21 -8], UCl, has been prepared in a similar fashion to UCl [10026-10-5] by reduction—chlorination of UO [1344-58-7] under flowing CCl, but at a lower temperature. Another synthetic approach which has been used is the oxidation of UCl by CI2. The pentachloride has been stmcturaHy characterized and consists of an edge-sharing bioctahedral dimer, U2CI2Q. The pentachloride decomposes in H2O and acid, is soluble in anhydrous alcohols, and insoluble in benzene and ethers. 

Uranium pentabromide [13775-16-1], UBr, is unstable toward reduction and the pentaiodide is unknown. Two synthetic methods utilized for the production of UBr involve the oxidation of uranium tetrabromide [13470-20-7], UBr, by Br2 or by bromination of uranium turnings with Br2 in acetonitrile. The metastable pentabromide is isostmctural with the pentachloride, being dimeric with edge-sharing octahedra U2Br2Q. 

Both U(I11) and U(IV) bis-Cp complexes have been isolated, with the former exhibiting slighdy lower stabiUties. Dimeric uranium(Ill) his-ring... 

Bismuth Penta.fIuoride, Bismuth(V) fluoride consists of long white needles that have been shown to have the same stmcture as the body-centered, tetragonal a-polymorph of uranium hexafluoride. The density of the soHd is 5.4 g/mL at 25°C. The soHd consists of infinite chains of trans-bridged BiF polyhedra dimers and trimers are present in the vapor phase (22). Bismuth pentafluoride may be prepared by the fluorination of BiF or... 

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. ° ... 

Diphenyl sulfoxide complexes of uranium(VI) have been reported (495) and a thorough infrared and conductivity study detailed (433), suggesting that the thiocyanate complexes of empirical formula [U02(R2S0)2(NCS)2] are in fact dimeric with bridging iV S-NCS ligands. 

The complexes MLa (M = Th, U) with HL = oc-, /J- or y-isopropyl tropolone are also known. Molecular weight determinations (benzene solution) for the thorium compounds indicate that the a compound is close to a dimer, whereas the 0 and y derivatives are approximately trimeric. The corresponding uranium(IV) complexes are all monomers. All three thorium compounds readily form monohydrates and the structure of the hydrated y-isopropyltropolonate is described on p. 1144. 

The dimeric complex (U02)2A2(HA2)2, reported to be present at low acidities,117 can be formulated similarly (structure 12). At high concentrations of uranium, polymers of increasingly long chains and of composition (U02) A2 2(HA2)2 are formed.118... 

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