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

The dithiophosphinic acids (HS₂PR₂) 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 (HS₂PPh₂) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS₂P(o-CF₃C₆H₄)₂], are employed to understand the substituent effect on the bonding covalency between the S₂PR₂^- anions (R = Ph and o-CF₃C₆H₄) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO₂(S₂PR₂)(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 \% S 3p character in these orbitals is essentially identical for the UO₂(S₂PPh₂)₂(EtOH) and UO₂[S₂P(o-CF₃C₆H₄)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 UO₂(S₂PR₂)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.