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)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 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)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, β = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729.