The coordination chemistry of actinides in aqueous solutions can be split into two groups: lower oxidation state (di, tri and tetravalent) and higher oxidation state (penta, hexa — and heptavalent) ions.

The coordination number and geometry of their aqueous complexes is determined by the electronic configuration and steric size and shape of the ligands. While ionicity is the predominant characteristic of both lanthanide and actinide bonding (Choppin, 2002), an appreciable covalency, stronger in the actinide bonds, has been confirmed by many spectroscopic studies and attributed to the 6d orbital interactions with the ligands, which are significantly stronger than the 5f interactions (Clark, 2006).

The actinide ions are relatively large cations. Their ionic radii range from 0.112 to 0.095 nm for the trivalent and from 0.094 to 0.082 nm for tetrava — lent cations. The actinides are found to have high coordination numbers, from 6 to 14. For a majority of the actinides, the exact numbers of water molecules that are bound to the metal centers in the hydrated metal ions are still controversial; the uncertainty in the structures can be explained by the limited number of crystal structures that exist for their aquo complexes. This lack of data is related to the difficulty in crystallizing materials from aqueous solutions (Keogh, 2005).

Typical examples of coordination geometries for An3+ and An4+ are the Structures 2.1-2.6 for their octa — and nona-aquo ions An(H2O)8/93+/4+. Structures (2.1-2.3) are octa-coordinate with a cubic, square antiprism, and a bicapped trigonal prism arrangement of the ligands (water molecules), respectively. The nine-coordinate structure (2.4) is a tricapped trigonal prism. Higher coordination numbers are observed with multidentate ligands, such as carbonate and nitrate. Ten-coordinate species include, for example, anionic pentacarbonate species of tetravalent actinides (Structure 2.5) with an irregular geometry with two trans carbonate ligands at the axial sites and three nearly planar carbonate ligands in a pseudo-equatorial plane that is reminiscent of the structure of dioxocation complexes of hexavalent actinides. The hexanitrate anion An(NO3)62- is extremely important in sepa­ration of actinides, for example plutonium, under conditions when other metals are in cationic form. Cation exchange resins have a strong affinity for the hexanitrato species Pu(NO3)62-. Structure 2.6 represents the coordi­nation geometry for this anion, which has six bidentate nitrate ligands, giving the central Pu4+ ion a coordination number of 12. A single crystal XRD study (Spirlet et al. ,1992) performed on (NH4)2Pu(NO3)6 confirmed

the structure (6) of the icosohedral Pu(NO3)62- unit characterized by three mutually perpendicular planes formed by the trans NO3- groups giving virtual symmetry Th. The twelve Pu-O bond distances average 2.487(6) angstroms (Clark, 200

For the aqueous species of penta — and hexavalent actinides, the typical species includes the linear dioxo unit, AnO2+/2+, with two oxygen atoms positioned at 180° with an average M-O distance of 0.175-0.180 nm for hexavalent cations and 0.181-0.193 nm for pentavalent cations. All “second­ary” ligands are coordinated in the perpendicular equatorial plane with typical M-X bond distances of 0.24-0.26 nm. The bonding for these ions has significant covalency with the axial An-O ligands, while the bonding for the majority of the ligands residing in the equatorial plane is primarily ionic (Keogh, 2005). As a result of this dual behavior (covalency and ionicity) of the trans dioxo ions, the linear dioxo unit is unperturbed (with the exception of bond distance changes) in all of the aqueous-based complexes. The coordination numbers of the central actinide cation are defined by the equatorial size of ligands and their electronic properties. The structures of a variety of aqueous-based coordination complexes have been observed (Structures 2.9-2.12). Compounds with tetragonal syStructure 2.9

Structure 2.12

The penta-aqua ion (Structure 2.11) and pentafluoro complex (Structure 2.12) for the hexavalent actinides are seven-coordinate structures, prevalent in actinide chemistry, and are the highest coordination numbers achievable with all monodentate ligands; however, coordination complexes with eight atoms bound to the actinide are achievable. The most well-studied aquo ion of the actinides is UO2(H2O)52+ (Structure 2.11). From EXAFS, structural data on the aquo ions have been obtained for the hexavalent ions, AnO2(H2O)52+ (An = U-Am). For calibration purposes, the bond dis­tance for the oxo ligands of the UO22+ species obtained from XAFS and single-crystal analyses show a nearly identical length. The bond An = O distance was found to be 0.176, 0.175; 0.174 and 0.18 nm for An(VI) = U, Np, Pu, and Am, respectively. The An-OH2 distance for the same complexes was found to be 0.242, 0.242, 0.241 and 0.24 nm, respectively (Keogh, 2005). For neptunyl and plutonyl aqua ions of pentavalent Np and Pu, 4, 5 and 6 water ligands were identified by XAFS. The bond distances for both An = O (0.183 nm for both Np and Pu) and An-OH2 (0.251 and 0.250, respec­tively) expand in the pentavalent ions in line with an increase in the ionic radii with the change in oxidation state.

Structures with 9-12 molecules of water have been proposed for tetrava — lent actinides in aqueous solutions. In general, the most accepted values for the number of H2O molecules bound to the metal center are 10 for Th and 9 for U to Pu. The An-OH2 distances in these ions range from 0.25 to 0.24 nm (Keogh, 2005). Trivalent plutonium with nine molecules of water (Matonic, 2001) was crystallized in a tricapped trigonal prismatic geometry.