Covalency of An(III) and Ln(III) can be explored using ligands containing soft donor atoms such as sulfur and selenium, whose orbitals should overlap more extensively with the radially extended 5f orbitals of actinides compared to the 4f orbitals of lanthanides. Combining these soft atoms into a mixed-donor ligand with hard, negatively charged oxygen atoms will provide strongly chelating ligands to test actinide/lanthanide covalency. Understanding this covalency may be key in determining how ligands can effectively separate An(III) and Ln(III) for purposes such as nuclear waste management, since the ionic radii and charges of these trivalent metals are nearly identical.<br/> <br/>We have recently developed a synthetic route toward previously inaccessible diacetate/dipropionate imidazole thiones and selones, which also enable investigation of the importance of pocket size and arm flexibility in the coordination. Mass spectrometry studies demonstrate coordination of these mixed O/S-donor ligands with Nd(III) in 1:1 and 2:1 ligand-to-metal stoichiometries. Indeed, aqueous phase extraction studies with Eu-152 from 1M HDEHP in heptane demonstrate stronger effective binding of our mixed O/S-donor ligands to Eu-152 than is observed for 2,5-thiophenedicarboxylic acid (TPA). Ongoing work exploring the extraction affinity of these ligands for Pu(III) is quantifying the selectivity between An(III) and Ln(III) and benchmark the strength of interaction of the <i>f</i>-block metal with the chalcogenic donor atom. Establishing this difference in the binding and extracting ability between the ligands will be important in demonstrating their utility in probing actinide covalency.

Probing trivalent actinide/lanthanide covalency through mixed O/S-donor imidazole thione ligands

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