Actinides carboxylic acid complexes

Actinide complexes with carboxylic acids. U. Casellato, P. A. Vigato and M. Vidali, Coord. Chem. Rev., 1978, 26, 85-159 (276). 

Table 37 Actinide(IV) Complexes with Carboxylic Acid Hydrazides, RCONHN=CHR1...

Structural aspects of the actinide carboxylate complexes have been reviewed235 from which the structures of many adducts that are likely to be involved in the extraction with carboxylic acid - neutral donor mixtures may be predicted. 

From fig. 1 it can be seen that in addition to the labile protein-bound fraction, metal-transferrin complexes in the case of actinides, there is also a low molecular mass fraction in which metal ions are bound to, and transported with, ligands such as the anions of amino and carboxylic acids. For the hard cations of the actinide elements, these complexing agents are predominantly carboxylic acids (from the citric acid cycle) (Duffield and Taylor 1986, Popplewell et al. 1975, Duffield et al. 1984, Metivier 1973) or inorganic anions such as carbonate, e.g., for uranium(VI) (Stevens et al. 1980). 

Robel, W. (1970) Complex Compounds of Hexavalent Actinides with Pyridine Carboxylic Acids, West German Report KFK-1070. 

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 ... 

The thermodynamic data for complexation of trivalent lanthanide and actinide cations with halate and haloacetate anions are reported. These data are analyzed for estimates of the relative amounts of inner (contact) and outer (solvent separated) sphere complexation. The halate data reflected increasing inner sphere character as the halic acid pKa increased. Use of a Born-type equation with the haloacetic acid pKa values allowed estimation of the effective charge of the carboxylate group. These values were, in turn, used to calculate the inner sphere stability constants with the M(III) ions. This analysis indicates increasing the inner sphere complexation with increasing pKa but relatively constant outer sphere complexation. 

Based on earlier research on the separation of the lanthanides, ion exchange separations were also done using citric add as a complexant. Because the citrate eluent is plagued by relatively slow kinetics, other hydroxy-carboxylic adds were tested as elutriants for the higher actinide elements. Lactic add was among the first spedes tried, but better interactinide separations were observed using a-hydroxyisobutyric acid... 

---