Chemical Speciation of Neptunium(VI) under Strongly Alkaline Conditions. Structure, Composition, and Oxo Ligand Exchange

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)₄^2– ions, each with pseudooctahedral coordination geometry with <i>trans</i>-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 L_III-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 ν₁(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 ν₁ 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)₄^2– is the dominant solution species under the conditions of study and that a small amount of a second species, NpO₂(OH)₅^3–, may also be present at higher alkalinity. Crystal data for [Co­(NH₃)₆]₂[NpO₂(OH)₄]₃·H₂O: monoclinic, space group <i>C</i>2/<i>c</i>, <i>a</i> = 17.344(4) Å, <i>b</i> = 12.177(3) Å, <i>c</i> = 15.273 Å, β = 120.17(2)°, <i>Z</i> = 4, R1 = 0.0359, wR2 = 0.0729