Probing Ni[S2PR2]2 Electronic Structure to Generate Insight Relevant to Minor Actinide Extraction Chemistry

A method to evaluate the electronic structure of minor actinide extractants is described. A series of compounds containing effective and ineffective actinide extractants (dithiophosphinates, S2PR2–) bound to a common transition metal ion (Ni2+) was analyzed by structural, spectroscopic, and theoretical methods. By using a single transition metal that provides structurally similar compounds, the metal contributions to bonding are essentially held constant so that subtle electronic variations associated with the extracting ligand can be probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information using analogous techniques with minor actinide and lanthanide complexes. Here, we demonstrate that this approach, supplemented with ground state and time-dependent density functional theory, provides insight for understanding why high separation factors are reported for the extractant HS2P­(o-CF3C6H4)2, while lower values are reported and anticipated for other HS2PR2 derivatives (R = C6H5, p-CF3C6H4, m-CF3C6H4). The implications of these results for correlating electronic structure with the selectivity of HS2PR2 extractants are discussed.