Structure, Spectroscopy, and Theoretical Analysis of Zero- And Three-Dimensional Lithium Plutonium Fluorides: Li₄PuF₈ and LiPuF₅

The reaction of ²⁴²PuO₂ with HF and LiF under hydrothermal conditions results in the formation of Li₄PuF₈ and LiPuF₅. These compounds were structurally characterized using single-crystal X-ray diffraction, and UV-vis-near-IR absorption spectroscopy was employed to confirm the oxidation state of the plutonium in the compounds as 4+. The structure of Li₄PuF₈ consists of [PuF₈]^4- anions that adopt a bicapped trigonal-prismatic geometry with approximate C_2v symmetry. These molecules are bridged by Li⁺ cations. In contrast, LiPuF₅ forms a dense three-dimensional network constructed from [PuF₉]^5- units that are bridged by F^- anions. The Pu⁴⁺ cations are found within tricapped trigonal prisms. Extensive theoretical analysis of the electronic and bonding interactions is included with a comparison between the results derived from complete-active-space self-consistent-field at different levels of theory, quantum theory of atoms in molecules, interacting quantum atom, natural localized molecular orbital, and Wiberg bond order analyses. Covalent interactions in these compounds are examined, and intramolecular trends in covalent and electrostatic interactions are discussed.