Covalency between the uranyl ion and di­thio­phosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory

The dithiophosphinic acids (HS(2)PR (2)) have been used for the selective separation of trivalent actinides (An(III)) from lanthanides (Ln(III)) over the past decades. The substituents on the di­thio­phosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two di­thio­phosphinic acids with significantly different An(III)/Ln(III) separation performance, i.e. di­phenyl di­thio­phosphinic acid (HS(2)PPh(2)) and bis­(ortho-trifluoro­methyl­phenyl) di­thio­phosphinic acid [HS(2)P(o-CF(3)C(6)H(4))(2)], are employed to understand the substituent effect on the bonding covalency between the S(2)PR (2) (−) anions (R = Ph and o-CF(3)C(6)H(4)) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO(2)(S(2)PR (2))(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of [Image: see text] S 3p character in these orbitals is essentially identical for the UO(2)(S(2)PPh(2))(2)(EtOH) and UO(2)[S(2)P(o-CF(3)C(6)H(4))(2)](2)(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U—S bonds for the two UO(2)(S(2)PR (2))(2)(EtOH) complexes are contradictory to the significantly different An(III)/Ln(III) separation performance of the two di­thio­phosphinic acids, thus the covalency seems to be unable to account for substituent effects in the An(III)/Ln(III) separation by the di­thio­phosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the An(III)/Ln(III) separation by the di­thio­phosphinic acids.