CHARACTERIZATION OF AQUEOUS PLUTONIUM(IV) NITRATE COMPLEXES BY EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY

The structures of the aqueous nitrate complexes of Pu(IV) have been st udied by extended X-ray absorption finestructure (EXAFS) at nitric aci d concentrations of 3, 8, and 13 M. Systematic changes in the EXAFS sp ectra demonstrate the trends of increasing nitrate ligation and decrea sing water ligation as a function of increasing nitric acid concentrat ion. The coordination numbers of the nitrogens and noncoordinating oxy gens are consistent with previous studies in which the principal speci es were found to be the di-, tetra-, and hexanitrato complexes in thes e solutions, The Pu(TV) nitrate complexes in nitric acid appear to hav e similar actinide-nitrate structures as seen in the solid state struc tures of analogous thorium and neptunium nitrate compounds in which th e nitrate ligands are planar and bidentate, and the central actinide a tom is in the plane of the nitrates. The nitrates are distorted with r espect to free nitrate in that the nitrogen-oxygen bond length of the noncoordinating oxygen is significantly shorter than the nitrogen-oxyg en bond lengths of the coordinated oxygens. The plutonium in these com plexes is highly coordinated with coordination numbers of 11-12 for th e first shell of oxygen nearest neighbors. The average plutonium-oxyge n (nitrate) bond length is 2.49 Angstrom, and the average plutonium-ox ygen (water) bond length is estimated to be 2.38 Angstrom.