The thermodynamics of complexation between hard acid cation and hard ligands are often characterized by positive values of both the enthalpy and entropy changes (Choppin, 2004). The positive enthalpy values indicate a net decrease in bond strength for the system (including the solvent) going
from reactants to products. The positive reaction entropy overcomes this unfavorable enthalpic component to promote complex formation. Many lanthanide and actinide complexation reactions are considered to be “entropy-driven”, since the entropy contribution from dehydration of cation is more significant than loss of entropy associated with the combina­tion of the ion with another ligand. The observed overall changes reflect the sum of the contributions of dehydration and cation-ligand combination. It has been shown that for many of the hard-hard complexation systems, there is a linear correlation between the experimental values of enthalpy and entropy of complexation reaction, a so called “compensation effect” (Choppin, 2004).

The hydration of an actinide cation is a critical factor in the structural and chemical behavior of their complexes. During the complexation reac­tion, one or more water molecules in the hydration sphere of the metal cation are replaced with a ligand, donating electrons and electrostatic mol­ecules to the central atom. Based on Pearson hard/soft acid/base theory, they are usually characterized as hard or soft donor-ligands. Actinides are “hard” Lewis acids and exhibit a strong preference for oxygen donor ligands. The complexation in aqueous solution many times involves substitution of the solvate waters with their metal—oxygen (ion-dipole) bond by a ligand (Choppin, 1971). The water expelling ligands can form either direct bonds with the metal cation in the first (inner) coordination sphere of the cation, creating a so called “inner-sphere complex”, or, if they cannot displace the water from their inner coordination sphere, they stay directly bonded only to the primary hydration sphere of the metal cation. Such “outer sphere ligands” remain separated from direct contact with the metal ion by a mol­ecule of water; such complexes have a very subtle influence on the behavior of the metal ion.

Actinide cations are known to form both outer and inner sphere com­plexes and, for labile complexes, it is often difficult to distinguish between these two types. Choppin proposed (Choppin, Strazik, 1965); (Ensor, Choppin, 1980) and (Khalili et al., 1988) the use of thermodynamic param­eters of complexation (enthalpy and entropy) to help evaluate outer sphere vs. inner sphere complexation. Because the primary solvation sphere is minimally perturbed by the ligand in outer sphere complexes, little energy is spent on de-solvation and little disordering occurs. As a result, outer sphere complexation is often associated with usually near zero enthalpy and negative and near zero entropy. In contrast, the enthalpy for inner sphere complexation is determined by the relative balance of metal-ligand bond strength and the metal-water bonds broken; usually it is small and maybe slightly exothermic while the entropy for inner sphere complexes is usually positive because more water molecules are released than ligands com — plexed (Nash, Sullivan et al., 1986, 1991).

Predominantly outer sphere ligands include Cl-, Br-, I-, ClO3-, NO3-, sulfonate, and trichloroacetate ligands, all with acid dissociation constant pKa values <2. As the pKa increases above 2, increasing predominance of inner sphere complexation is expected for carbonate/bicarbonate, sulfate, fluoride, and most carboxylate ligands (Choppin, 1998). Comparison of the thermodynamic parameters of nitrates and chlorides also reflect the inter­mediate character of the nitrate complexes (Choppin, Strazik, 1965; Choppin, Graffeo, 1965); a near zero entropy suggests that the nitrate is probably mainly monodenate with one water molecule displaced. Water coordination may induce changes in anion coordination mode and coordi­nation number. Quantum mechanical calculations suggest that when the first coordination shell is saturated, the two types of binding modes become of similar energy, leading to different coordination numbers (CNs) and distributions of first and second shell water molecules. For instance, for La(NO3)3(H2O)6, CN ranges from 9 (3 monodentate nitrates + 6 water) to 10 (3 bidentate nitrates + 4 water) or 11 (3 bidentate nitrates + 5 water). Thus, at some point, adding water to the second or to the first shell becomes isoenergetic. As the cation becomes smaller, the preference for monoden­tate nitrate binding increases, due to avoided repulsions in the first coordi­nation sphere (Dobler et al., 2001).

Whether the U(VI)-nitrate complex is “outer sphere” or “inner sphere” is another question that is still open for debate (Rao, Tian, 2008). Earlier data on the complexation of U(VI) with nitrate at variable temperatures (10-40°C) appeared to suggest that the UO2NO3+ complex was outer sphere (Ensor, Choppin, 1980; Khalili et al., 1988). However, Rao’s calorimetric data suggest that both the inner and outer sphere complexes may exist in the U(VI) nitrate system and the UO2NO3+ complex has significant inner sphere character (Rao, Tian, 2008).

The structure of actinide complexes, when H2O is replaced by the addi­tion of the ligand, for example OH-, F-, CO32-, tends to weaken the v1 stretch of the O = An = O and increase the An = O distance. On transitioning between AnO2+ to An4+: the loss of the linear dioxo unit in the lower valent complexes results in both a higher overall charge and a smaller radius of the cation, which combines to result in a slight decrease in the An-ligand bond distance (Keogh, 2005), and with a stronger metal-ligand interaction.

Experimental data on the size and structure of the hydration sphere of a metal ion are very important for understanding of metal complexation and the behavior of complexes in separation systems. They have been probed by several direct and indirect experimental methods. Recently, two reviews devoted to critical evaluation of results obtained for solution coor­dination chemistry of actinides by both direct, including X-ray and neutron diffraction, X-ray absorption fine structure (XAFS) measurements, lumi­nescence decay, nuclear magnetic resonance (NMR) relaxation measure­ments, and indirect methods such as compressibility, NMR exchange, and optical absorption spectroscopy, have been published (Choppin, Jensen, 2006; Szabo et al., 2006) Theoretical and computational studies are also important in understanding the coordination geometry and coordination number (CN) of actinide ion hydrates (e. g., Spencer et al., 1999; Hay et al., 2000; Tsushima, Suzuki, 2000; Antonio et al., 2001).