Interactions with actinide elements involve both the 5f and 6d orbitals, and bonding is often intermediate between covalent and ionic with significant relativistic contributions. The extent to which actinide elements engage in covalent bonding with light atoms is still uncertain, and determining the relative roles of the 5f and 6d orbitals is a considerable experimental and computational challenge. The exact nature of chemical bonds between actinides and light atoms (C, N, O, F) is of particular interest because these interactions control many physical, chemical, and biological processes involving actinides. Light atoms are able to form highly covalent actinide–ligand multiple bonds, however, increasing 5f orbital covalency does not always result in a concomitant increase in bond stability. Among approaches explored previously, X-ray absorption spectroscopy (XAS) conducted with a scanning transmission X-ray microscope has emerged as an effective method for quantitatively probing electronic structure and orbital mixing. Recent advances in theory have also significantly improved descriptions of ground state physical properties and calculations of core-hole spectra. This presentation will describe our recent efforts to use inorganic synthesis, ligand K-edge XAS, and calculations to evaluate the relative roles of 5f and 6d orbitals in actinide molecules and extended solids. Our efforts began with O K-edge studies of the actinyl tetrachloride ions, AnO2Cl42- (An = U, Np, Pu), which represent the most important high-valent coordination compounds in actinide science. The transuranic neptunyl and plutonyl compounds were prepared in specialized radiochemical synthesis facilities at Lawrence Berkeley National Laboratory. Characterization using O K-edge XAS at the Advanced Light Source provided a direct experimental observation of a structural inverse trans influence resulting from hybridization of the An 5f orbitals with the 6p pseudocore. O K-edge studies of the lanthanide analogs will also be discussed, including the dioxides and sesquioxides, LnO2 (Ln = Ce, Pr, Tb) and Ln2O3 (Ln = La to Lu). In this instance, changes in the amount of O 2p and Ln 5d mixing were closely correlated to the well-established trends in the chemical accessibility of the 4f orbitals towards oxidation or reduction. The unique insight provided by ligand K-edge XAS will be discussed in the context of known differences in lanthanide and actinide chemical reactivity and physical properties.

Synthetic, spectroscopic, and theoretical investigations into the roles of the 5f and 6d orbitals in actinide–ligand bonding

ACS Spring 2022

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