Comparison of Americium(III) and Neodymium(III) Monothiophosphate Complexes

Mixed-donor ligands, such as those containing a combination of O/N or O/S, have been studied extensively for the selective extraction of trivalent actinides, especially Am³⁺ and Cm³⁺, from lanthanides during the recycling of used nuclear fuel. Oxygen/sulfur donor ligand combinations also result from the hydrolytic and/or radiolytic degradation of dithiophosphates, such as the Cyanex class of extractants, which are initially converted to monothiophosphates. To understand potential differences between the binding of such degraded ligands to Nd³⁺ and Am³⁺, the monothiophosphate complexes [M(OPS(OEt)₂)₅(H₂O)₂]^2- (M³⁺ = Nd³⁺, Am³⁺) were prepared and characterized by single-crystal X-ray diffraction and optical spectroscopy and studied as a function of pressure up to ca. 14 GPa using diamond-anvil techniques. Although Nd³⁺ and Am³⁺ have nearly identical eight-coordinated ionic radii, these structures reveal that while the M-O bond distances in these complexes are almost equal, the M-S distances are statistically different. Moreover, for [Nd(OPS(OEt)₂)₅(H₂O)₂]^2-, the hypersensitive ⁴I_9/2 → ⁴G_5/2 transition shifts as a function of pressure by -11 cm^-1/GPa. Whereas for [Am(OPS(OEt)₂)₅(H₂O)₂]^2-, the ⁷F₀ → ⁷F₆ transition shows a slightly stronger pressure dependence with a shift of -13 cm^-1/GPa and also exhibits broadening of the 5f → 5f transitions at high pressures. These data likely indicate an increased involvement of the 5f orbitals in bonding with Am³⁺ relative to that of Nd³⁺ in these complexes.