Complexes of the trivalent actinides

Although no single crystal X-ray work has been done on the cyclopentadienide complexes of the trivalent actinides, it is clear that they have structures similar to those of the known homologous lanthanides. Both the trivalent lanthanides and actinides behave as Lewis acids and form adducts to complete their coordination spheres. An optimum formal coordination number of ten is indicated and their structures seem to be dictated by a maximization of electrostatic interactions within the steric constraints of the ligands. 

Only six 261Rf events were observed in over one hundred experiments, one in the feed fraction (12 M HC1), two in elution fraction 2 (6 M HC1), and three in elution fraction 3 (6 M HC1). The percentage of Hf in these same fractions was 12%, 59%, and 29%. These results showed the chloride complexation of Rf is consistently stronger than that of the trivalent actinides and is similar to that of Hf. Again, no Kd value was determined in this early experiment. 

The LiCl AIX process is based on (i) the formation of anionic chloride complexes of the tripositive actinide and lanthanide metals in concentrated LiCl solutions, (ii) the sorption of these complexes onto a strong base anion exchange resin contained in a column, and (iii) the preferential chromatographic elution of the lanthanides as a group prior to elution of the actinides. The generalized formation of the trivalent metal anionic chloride complexes is illustrated in equation (1) ... 

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. ... 

Further studies of the spectra of the trivalent actinide hexahalide complexes are being carried out and will be presented in greater detail later. 

XAFS data for complexes of Cm with the HC301 extractant indicate only sulfur donation to the metal in the inner sphere of coordination. HC301 forms 3 1 complexes with the trivalent actinides and are coordinated in a bidentate mode as seen in Figure 91. Data indicates a hexacoordinate structure that resembles Z>3 symmetry in lanthanide dithiophosphinic acid complexes. 

The first computer-controlled automated system for performing very rapid solution chemistry experiments on an atom-at-a-time basis was used in later pioneering experiments (Hulet et al., 1980). Results showed that the anionic-chloride complexes of Rf were clearly similar to Hf and much stronger than those of the trivalent actinides, again confirming the position of Rf in group 4 of the periodic table. 

While the idea that ligand 7 could prove useful for the coordination of other, non-uranyl actinide cations, has yet to be tested by experiment, it is important to note that this ligand has so far proved less than satisfactory for the coordination of other large, non-actinide cations. Indeed, in spite of extensive efforts devoted to the problem, no stable, non-labile complexes of the trivalent lanthanides (ionic radii 0.86 - 1.36 A ) have as yet been characterized with this system. Nor have 1 1 complexes with other large cations, e.g., Cd + (ionic radius 1.0 A ) or Pb + (ionic radius 1.2 A ), been documented.This has proven to be the case even though mass spectrometric evidence consistent with metal coordination has been obtained in certain instances. 

In conclusion, we believe that the differences between the complexes of actinides studied and those of the corresponding lanthanides are due to other contributing factors in the interaction between central ion and ligand rather than to structural differences. Thermodynamic data on complexes of other trivalent actinides as well as study of homologous chelating agents would of course be necessary to understand better the phenomena. 

The trisolvate of the trisnitrato complex of the trivalent metal ion is extracted into the organic phase (yielding a formally nine-coordinate hydrophobic complex assuming nitrate is bidentate). Unlike the extraction of the tetravalent and hexavalent actinides, extremely high concentrations of nitric acid are required for even moderate extraction of the trivalent metal ions. Greater efficiency is observed for this system if alkali-metal salts are used as the aqueous medium as discussed below. 

In further ion-exchange experiments using solvent extraction chromatography, Hulet and co-workers investigated the chloride complexation of element 104 and compared it to that of the actinides and Hf [80]. The anionic chloride complexes of 104 were compared to those of Hf, Cm, and Fm by testing their relative adsorbabilities onto a column containing a quaternary amine. The results showed that in 12 M HCl solutions the chloride complexation of element 104 was clearly stronger than those of the trivalent actinides and quite similar to that of Hf. 

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. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

The overall distribution of lanthanides in bone may be influenced by the reactions between trivalent cations and bone surfaces. Bone surfaces accumulate many poorly utilized or excreted cations present in the circulation. The mechanisms of accumulation in bone may include reactions with bone mineral such as adsorption, ion exchange, and ionic bond formation (Neuman and Neuman, 1958) as well as the formation of complexes with proteins or other organic bone constituents (Taylor, 1972). The uptake of lanthanides and actinides by bone mineral appears to be independent of the ionic radius. Taylor et al. (1971) have shown that the in vitro uptakes on powdered bone ash of 241Am(III) (ionic radius 0.98 A) and of 239Pu(IV) (ionic radius 0.90 A) were 0.97 0.016 and 0.98 0.007, respectively. In vitro experiments by Foreman (1962) suggested that Pu(IV) accumulated on powdered bone or bone ash by adsorption, a relatively nonspecific reaction. On the other hand, reactions with organic bone constituents appear to depend on ionic radius. The complexes of the smaller Pu(IV) ion and any of the organic bone constituents tested thus far were more stable (as determined by gel filtration) than the complexes with Am(III) or Cm(III) (Taylor, 1972). 

The trivalent actinide state resembles that of the lanthanides. In an aqueous solution some M3+ ions exist (Am3+, Cm3+) ions the U3+ ions is readily oxidised by air or more slowly by water. Tetravalent U and Pu are reasonably stable in solution, whereas Am(IV) and Cm(IV) are readily reduced and exist only as complex ions in... 

These considerations lead, for example, to the assignment of a predominantly outer sphere character to Cl, Br, F, CIO3, NO3, sulfonate, and trichloro-acetate complexes and an inner sphere character to F", IO3, SO, and acetate complexes of trivalent actinides and lanthanides. The variation in AH° and AS° of complexation of related ligands indicates that those whose pA), values are <2 form predominantly outer sphere complexes, while those for whom > 2 form predominantly inner sphere complexes with the trivalent lanthanides and actinides. As the pK increases above 2, increasing predominance of inner sphere complexation is expected for these metals. 

As in the SETFICS and TALSPEAK processes, the DIAMEX-SANEX/HDEHP process involves selectively back-extracting the trivalent actinides by a hydrophilic polyamino-carboxylate complexing agent, HEDTA, in a citric acid buffered solution (pH 3). However, the combination of HDEHP and DMDOHEMA at high acidity promotes the coextraction of some block transition metals, such as Pd(II), Fe(III), Zr(IV), and Mo(VI), which must be dealt with by specific stripping steps (as described on Figure 3.26) that increase the total volume of the output streams ... 

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