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Nitrilotriacetic acid, commonly known as NTA (N(CH(2)CO(2)H)(3)), can be considered a representative of the polyaminocarboxylic family. The results presented in this paper describe the thermodynamical complexation and structural investigation of An(IV) complexes with NTA in aqueous solution. In the first part, the stability constants of the An(IV) complexes (An = Pu, Np, U, and Th) have been determined by spectrophotometry. In the second part, the coordination spheres of the actinide cation in these complexes have been described using extended X-ray absorption fine structure spectroscopy and compared to the solid-state structure of (Hpy)(2)[U(NTA)(2)] x (H(2)O). These data are further compared to quantum chemical calculations, and their evolution across the actinide series is discussed. In particular, an interpretation of the role of the nitrogen atom in the coordination mode is proposed. These results are considered to be model behavior of polyaminocarboxylic ligands such as diethylenetriamine pentaacetic acid, which is nowadays the best candidate for a chelating agent in the framework of actinide decorporation for the human body.

To fine-tune the design of optimized donor ligands for nuclear waste actinide selective extraction, both electronic and molecular structures of the actinide complexes that are formed must be investigated. In particular, to achieve the selective complexation of transplutonium 3+ ions versus lanthanide 3+ ions is one of the major challenges, given the chemical similarities between these two f-element families. In this work, the structure of solvent-phase M(NO3)3(TEMA)2 complexes (Ln = Nd, Eu, Ho, Yb, Lu, Am; TEMA = N,N,N',N'-tetraethylmalonamide) was investigated by liquid-phase spectroscopic methods among which extended X-ray absorption fine structure played a major role. In addition, the crystal structures of the species Nd(NO3)3(TEMA)2 and Yb(NO3)3(TEMA)2 have been determined by X-ray diffraction. Nd(NO3)3(C11N2O2H22)2 crystallizes in the monoclinic system (P2(1) space group; a = 11.2627(4) A, b = 20.5992(8) A, c = 22.2126(8) A; alpha = gamma = 90 degrees, beta = 102.572(1) degrees; Z = 6), and Yb(NO3)3(C11N2O2H22)2 crystallizes in the orthorhombic system (P2(1)2(1)2(1) space group; a = 9.3542(1) A, b = 18.1148(2) A, c = 19.7675(2) A; alpha = beta = gamma = 90 degrees; Z = 4). In the solvent phase, the metal polyhedron was found to be similar to that of the solid-state complex Nd(NO3)3(TEMA)2 for M = Nd to Ho. For M = Yb and Lu, a significant elongation of one nitrate oxygen bond was observed. Comparison with measurements on the Am(NO3)3(TEMA)2 complex in ethanol has shown the similarities between the Nd3+ and Am3+ coordination spheres.