As the electrolyte concentration increases, the number of water molecules in the secondary hydration sphere decreases. Consequently, there is a tight­ening of the bond between the metal cation and the hydrate waters in the inner sphere (Choppin, Jensen, 2006). Based on NMR studies of trivalent actinides and lanthanides, Choppin concluded that inner sphere complex — ation by perchlorate ions does not occur below approximately 8-10 M (Choppin, Labonne-Wall, 1997). Multiple equilibria for the uranyl chloride system (UO2Cl2(H2O)2, UO2Cl3(H2O)-, and UO2Cfi2-) have been used for separation of uranium from its progeny or other metals. Since Th4+ does not form anionic chloride complexes, it is retained on cation-exchange resin while anionic chloride complexes of UO22+ pass through the column in the eluate. Alternatively, such anionic complexes can be retained on an anion — exchange column.

The hydration number of Eu(III) remains relatively constant in hydro­chloric acid up to approximately 6-8 M, above which concentration it decreases. The same is true for the hydration number of Cm(III) in HCl, which begins a decline at about 5 M HCl. This difference between (Eu3+ and Cm3+) reflects greater complexation of the actinide trivalent ion by the relatively soft anion Cl-. The difference in chloride complexation has been used to provide efficient separation of trivalent actinides from trivalent actinides in concentrated HCl solutions by passage through columns of cation exchange resin since 1950s (Diamond et al., 1954).

Nitrate complexes for tetravalent actinides, for example, Th4+ and Pu4+, are extremely important in actinide separation and purification processes. Nitrate ions begin to form inner sphere complexes at lower concentrations than chloride anions; this observation is confirmed by the decreased hydra­tion number of the cation even at relatively lower concentrations (Choppin, Jensen, 2006). However, since the oxygen atoms of the nitrate are hard donors, there is no evidence of any covalent enhancement in its bonding as is seen with the chloride anions for the trivalent actinide cations relative to the lanthanide cations (Choppin, Jensen, 2006). In separation and puri­fication processes, the nitrate complexes of actinides are extremely impor­tant. Nitrate-nitric acid solution is the most common aqueous medium in nuclear separation processes. In the case of neutral extractants such as tributylphosphate (TBP), carbamoyl methyl phosphine oxide (CMPO) or dipicolinamides (DPA) it provides nitrate units necessary to compensate the actinide cation charge to enable extraction. Nitrate complexation with hexavalent actinide ions is very weak and the determination of the forma­tion constants for aqueous nitrate solution species is extremely difficult. Under aqueous conditions with high nitric acid concentrations, complexes of the form AnO2(NO3)(H2O)x+, AnO2(NO3)2(H2O)2, and AnO2(NO3)3- (An = U, Np, Pu) are likely to be present. The limiting species in the nitrate series is the hexanitrato complex, An(NO3)62- (Matonic et al, 2002). The complexation of the Pa and Np pentavalent ions by nitrate is known; however, limited thermodynamic and structural data are available. The presumed stoichiometry for the Np(V) species is NpO2(NO3)(H2O)x. For protactinium, which easily hydrolyzes, mixed hydroxo/nitrato or oxo/nitrato complexes have been proposed.

Fluorides and chlorides are the best studied actinide-halide systems, and they are very important for the pyroprocessing and electrorefining processes.

Carboxylic acids are strongly bound to actinide ions. The primary binding mode for simple carboxylic acids is bidentate, while in polycarboxylic acid complexes, carboxylates tend toward monodentate coordination with the metal ion. The affinity of the low-valent actinides for these ligands increases with the denticity of the ligand, for example, ethylenediaminetet — raacetate (EDTA) >>> acetate. For An4+, the EDTA ligand is hexadentate with a twist conformation (a spiral conformation, wrapping around the metal ion, rather than encapsulating the metal ion in a central cavity in the manner of tripodal or macrobicyclic ligands). Diethylenetriamine — N, N,N’,N",N"-pentaacetate (DTPA) has an even higher affinity for both

An3+ and An4+ ions.