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Hexavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li+, Na+, NMe4+ = TMA+) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)6Cl3 and NpO22+ in 3.5 M [N(Me)4]OH to 5 °C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)6]2[NpO2(OH)4]3·H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)42- ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np＝O and Np-OH distances were determined to be 1.80(1) and 2.24(1) Å, respectively. EXAFS data and fits at the Np LIII-edge on solid [Co(NH3)6]2[NpO2(OH)4]3·H2O and aqueous solutions of NpO22+ in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)6]2[NpO2(OH)4]3·H2O reveal a v1(O＝Np＝O) symmetric stretch at 741 cm-1. Raman spectra of NpO22+ recorded in a 0.6-2.2 M LiOH solution reveal a single v1 frequency of 769 cm-1. Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with 16O- and 18O-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)42- is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)53-, may also be present at higher alkalinity. Crystal data for [Co(NH3)6]2[NpO2(OH)4]3·H2O: monoclinic, space group C2/c, a = 17.344(4) Å, b = 12.177(3) Å, c = 15.273 Å, ß = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729. [ABSTRACT FROM AUTHOR]

The trivalent lanthanide bis-hydroaychlotlde compounds, Ln(OH)2Cl, (Ln = Nd through Lu, with the exception ot Pm and Sm) have been prepared by hydrothermal synthesis starting with LnCI3 · nH2O. Theae compounds were synthesized at temperatures not exceeding the melting point of the Teflon liners in the Parr autoclaves (∼220 °C). The compounds obtained were characterized by single crystal X-ray diffraction analysis, diffuse reflectance, FT-IR, and FT-Raman spectroscopies. Most of the lanthanide(lll) bis-hydroxychiorides are isostructural and generally crystallize in the monoclinic space group P21/m. The bis-hydroxychtorides of the heavier lanthanide(lll) atoms with smaller ionic radii also crystallize in the orthorhombic crystal system. Apparently hydrogen bonds between the OH groups and the Cl atoms connect the layers in the "c" direction. These H-bonds seem to be the driving force for the angle β of the monodinic complexes to decrease with decreasing ionic radius of the Ln(lll) ion and also for tying the layers together more strongly. As a result of this behavior, the structure of the heavier 41 analogues significantly resembles that of their orthorhombic counterparts. The heavier lanthanide bis-hydroxychlorides generally crystallize in the orthorhombic modification. The IA absorbance and Raman frequendes of the hydroxide ligands correlate as a function of the central lanthanide(lll) ionic radius. This observation is corroborated by X-ray diffraction (XRD) structural data. These compounds are quite insoluble in near-neutral and basic aqueous solutions, but soluble in acidic solutions. It is expected that the analogue actinide bis-hydroxychiorides exhibit similar behavior and that this may have important implications in the immobilization and safe disposal of nuclear waste. [ABSTRACT FROM AUTHOR]