Actinide anionic complexes

Uranyl carbonato complexes are important and much studied since they occur in Nature and are of environmental concern this is also the case for other actinide carbonate complexes.40 The C03 ion is exceptionally strongly bound to UO + and similar actinide ions. There are several naturally occurring minerals such as U02C03 while the anion [U02(C03)3]4 has importance in the extraction of U from ores and is responsible for the migration of U02+ in ground waters. Interaction of the 4-ion with HC104 proceeds as follows ... 

Trivalent. The trivalent actinides form simple carbonates of the formula Au2(C03)3. When these solids are dissolved in aqueous solutions of alkali metal carbonates, anionic complexes are formed, for example An(C03) " (n = 2-4). 

As with the case of the trivalent actinides, the tren, trisamidoamine (NNj), ligand has been used to stabilize An complexes. The preparation of [An(NN) )C1]2 is accomplished by the reaction of the trilithium salt of NNj with AnCLt (An = Th, U). The chloride ligand can be exchanged using metathetical reactions to form AnlTSINjlX (X = Br, I, NR2, OR). Anionic complexes, An(NN))XX (X = OR, X = OR ) can also be formed by addition of alkoxide salts to the neutral species. Complexes of Arf with diamidoamine ligands have also been studied. ... 

A wide variety of anionic actinide halide complexes are well known and typically are isolated with alkah or aUcahne-earth metal ions. The tendency and stability ofthe anionic complexes follow the trend F > Cl > Br I. The bivalent fluorides and... 

The trivalent actinides, like the trivalent lanthanides, form only weak chloride complexes in aqueous solution, and although there is evidence of slight formation of anionic complexes in concentrated LiCl from anion exchange data, no anionic chloride complexes have previously been positively identified. Ryan and j0rgensen have recently prepared the trivalent lanthanide hexachloro and hexabromo complexes and studied their absorption spectra. This paper discusses preliminary results of the extension of this work to the trivalent actinides. 

Spectrophotometric studies were used to identify the anionic hexavalent actinide acetate complexes present in aqueous and nonaqueous solutions, anion exchange resins, and amine extracts. The previously unreported tetraacetate complexes, M02(CJd 0i),, were identified in all these systems. The formation constant for the reaction +... 

The triacetate uranyl complex (24) is structurally similar to the trinitrate uranyl complex (6) (three bidentate ligands arranged equa-torially around the uranyl O—U—O axis). It was expected that the visible, near ultraviolet spectrum of the triacetate uranyl complex would be similar to the spectrum of U02(N03)3 as are the spectra of 1102(804)3, 1102(003)3 , and other uranyl complexes which apparently have the same structure (17). The absorption spectrum of uranyl acetate extracted into tri-n-octylamine in xylene from dilute acetic acid is different from the trinitrato uranyl spectrum. This indicates that the triacetate uranyl complex is probably not the species involved. By analogy to the uranyl nitrate system 14), formation of a tetraacetate uranyl complex might be expected. The purpose of this work is to determine the nature of the anionic hexavalent actinide acetate complexes and to identify the species involved in the amine extraction and anion exchange of the hexavalent actinides from acetate systems. 

The actinide (VI) acetate system discussed here and the actinide (VI) sulfate system to be discussed later (17) represent the only cases known to us in which observable mixtures of labile anionic complexes of a given metal are extracted from an acid solution by susbtituted amines. The present system represents the only one in which the diluent is the only major factor controlling the ratio of species in the organic phase. The principle mechanism by which the nature of the diluent appears to determine the ratio of species in the organic phase in this system is by... 

Aside from the neutral tris(amido)actinide complexes that have been prepared with sterically encumbering ligands as described, an alternate approach to the stabilization of trivalent actinide amides is the generation of anionic ate -type complexes. As an example, reaction of Ul3(THF)4 with excess KHNAr (Ar = 2,6-Pr 2CgH3) produces the anionic complex [K(THF)2]2[U(NHAr)5] which has been crystallographically characterized. ... 

Tetravalent. The best-studied tetravalent actinide carbonate complex is An(C03)5 (An = Th, U, Pu). This anion has been isolated using a variety of cations, including Na+, K+, T1+, [Co(NH3)s]3+ and C(NH2)3+/NH4+. In solution, the pentacarbonato complex is the end member of the series An(C03) " (n = 1-5) however, in the mineral tuliokite, Na6BaTh(C03)6-6H20, thorium exists as a hexacarbonato complex. The analysis of the thermodynamic data for these actinide carbonate systems has led to differences of opinion on the actual speciation. The data appear to support both the stepwise addition of COs and subsequent loss of H2O molecules within the An + cation coordination sphere as well as the formation of mixed hydroxo carbonate complexes, for example Pu(C03)3(0H). ... 

A wide variety of anionic actinide halide complexes are well known and typically are isolated with alkali or alkahne-earth metal ions. The tendency and stability of the anionic complexes follow the trend F Cl > Br I. The trivalent fluorides and chlorides typically form complexes of the form AnX4 and AnXe ". Plutonium has also been shown to give the following complexes PuCb ", Pu2Cl7, and PuClg . The anionic tetravalent actinide fluorides represent a broad class of complexes, for example, M AnFj, (x = 1, y = 5 x = 2, y = 6 V = 3, y = 7 x = 4, y = 8). Tetravalent actinide chloro, bromo and iodo complexes can be isolated from aqueous solutions in the form of octahedral AnCle " ions. 

The pentahalides of Pa and U generally add another ligand to form octahedral complexes of the form AnXg . The fluorides of Np and Pu have been isolated as AnFg , AnFy and AnFs . The predominate anionic complexes for the hexavalent actinides are based on UFg. Uranium hexafluoride will add up to two additional fluoride ions to form UFy andUFs using standard techniques. The unique UFg ion is formed from the thermal decomposition of Na2UFs. 

U(C2B9Hii)2Cl2] is the first reported actinide metallocarbaborane complex. It results from the reaction of UCU in THF with the 1,2-dicarbollide anion. The crystal structure shows the U atom to have approximately tetrahedral geometry, with pentahapto-dicdtrhoUide ligands (Figure 13). ... 

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