Actinides sulfate complexes

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... 

Polymerization of coordination polyhedra of actinides and sulfate anions does not always result in 2D topologies. Fig. 13a shows the Np" coordination in the structure of NaioNp2(S04)9(H20)4 [62]. The coordination sphere of the Np" ion includes eight O atoms of SO4 groups and one H2O molecule. The Np polyhedra and SO4 tetrahedra share common O atoms to produce the complex chains shown in Fig. 13b. 

Complex sulfates of the type MAn(S04)2 -XH2O, where M = alkali metal, have been precipitated from solutions of M2SO4 and the appropriate trivalent actinide ion in dilute HCl or H2SO4 [158]. Structural characterization is lacking for these compounds. 

DeCarvalho and Choppin (10, 11) previously have reported the stability constants, complexation enthalpies, and entropies for a series of trivalent lanthanide and actinide sulfates. As their work was conducted a pH 3, the dominant sulfate species was S0 and the measured reaction was as in equation 12. 

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

Amines, hydrazines, and hydroxylamines. Amine complexes are known for tetravalent complexes of the earliest actinides (Th, U), particularly for the halides, nitrates, and oxalates. The complexes are generated either in neat amine, or by addition of amine to the parent compound in a nonaqueous solvent. Some of the known simple amine compounds are presented in Table 6. The molecular structure of ThCl4(NMe3)3 has been determined. The coordination environment about the metal is a chloride capped octahedron. A very limited number of adducts exist in which a tetravalent actinide is coordinated by a hydrazine or hydroxylamine ligand the parent compound is generally a halide or sulfate complex. Cationic metal hydrates coordinated with primary, secondary, or tertiary amines have also been isolated with acetylacetonate, nitrate, or oxalate as counterions. 

Sulfates and sulfites. Mono- and bis-sulfate complexes of actinyl ions, An02S04 and An02(S04)2 , are generally prepared from acidic solutions. The geometry about the actinide metal center is pentagonal biypyramidal from actinyl, sulfato, and aquo oxygen atoms. A tris(sulfato) complex has been reported, but it is very weak if it does exist. Ternary hydroxo, sulfato complexes have been reported for uranyl, but they have not been structurally characterized. 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

Salts of 0x0 anions, such as nitrate, sulfate, perchlorate, iodate, hydroxide, carbonate, phosphate, oxalate, and so on, are important for the separation and reprocessing of technologically important actinides, while hydroxide, carbonate, and phosphate iorts are important for the chemical behavior of the actinides in the environment. The general trends of complexes formed in aqueous solutions are as follows ... 

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