Characterization of Aqueous Plutonium(IV) Nitrate Complexes by Extended X-ray Absorption Fine Structure Spectroscopy

• Published in: Inorganic chemistry 1996-05, Vol.35 (10), p.2841-2845
• Main Authors: Allen, P. G, Veirs, D. K, Conradson, S. D, Smith, C. A, Marsh, S. F
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic9511231

The structures of the aqueous nitrate complexes of Pu(IV) have been studied by extended X-ray absorption fine-structure (EXAFS) at nitric acid concentrations of 3, 8, and 13 M. Systematic changes in the EXAFS spectra demonstrate the trends of increasing nitrate ligation and decreasing water ligation as a function of increasing nitric acid concentration. The coordination numbers of the nitrogens and noncoordinating oxygens are consistent with previous studies in which the principal species were found to be the di-, tetra-, and hexanitrato complexes in these solutions. The Pu(IV) nitrato complexes in nitric acid appear to have similar actinide−nitrate structures as seen in the solid state structures of analogous thorium and neptunium nitrate compounds in which the nitrate ligands are planar and bidentate, and the central actinide atom is in the plane of the nitrates. The nitrates are distorted with respect to free nitrate in that the nitrogen−oxygen bond length of the noncoordinating oxygen is significantly shorter than the nitrogen−oxygen bond lengths of the coordinated oxygens. The plutonium in these complexes is highly coordinated with coordination numbers of 11−12 for the first shell of oxygen nearest neighbors. The average plutonium−oxygen (nitrate) bond length is 2.49 Å, and the average plutonium−oxygen (water) bond length is estimated to be 2.38 Å.