First-principles study of the separation of Am(III)/Cm(III) from Eu(III) with Cyanex301

The experimentally observed extraction complexes of trivalent lanthanide Eu(III) and actinide Am(III)/Cm(III) cations with purified Cyanex301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid, HBTMPDTP denoted as HL], i.e., ML(3) (M = Eu, Am, Cm) as well as the postulated complexes HAmL(4) and HEuL(4)(H(2)O) have been studied by using energy-consistent 4f- and 5f-in-core pseudopotentials for trivalent f elements, combined with density functional theory and second-order Møller-Plesset perturbation theory. Special attention was paid to explaining the high selectivity of Cyanex301 for Am(III)/Cm(III) over Eu(III). It is shown that the neutral complexes ML(3), where L acts as a bidentate ligand and the metal cation is coordinated by six S atoms, are most likely the most stable extraction complexes. The calculated metal-sulfur bond distances for ML(3) do reflect the cation employed; i.e., the larger the cation, the longer the metal-sulfur bond distances. The calculated M-S and M-P bond lengths agree very well with the available experimental data. The obtained changes of the Gibbs free energies in the extraction reactions M(3+) + 3HL → ML(3) + 3H(+) agree with the thermodynamical priority for Am(3+) and Cm(3+). Moreover, the ionic metal-ligand dissociation energies of the extraction complexes ML(3) show that, although EuL(3) is the most stable complex in the gas phase, it is the least stable in aqueous solution.