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)22– (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)22–, the hypersensitive 4I9/2 → 4G5/2 transition shifts as a function of pressure by −11 cm–1/GPa. Whereas for Am(OPS(OEt)2)5(H2O)22–, 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.