Covalency in Americium(lll) Hexachloride

摘要

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficuldy in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers (Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954, 76, 1461) when it was proposed that covalency from 5/-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding in—arguably—one of the most difficult systems to study, the Am—CI interaction within AmCl_6~(3-). We observed both 5/- and 6d-orbital mixing with the Cl-3p orbitals; however, contributions from the 6d-orbitals were more substantial. Comparisons with the isoelectronic EuCl_6~(3-) indicated that the amount of CI 3p-mixing with Eum 5d-orbitals was similar to that observed with the Am~Ⅲ 6d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that Am~Ⅲ 5f-orbital covalency was more substantial than 4f-orbital mixing for Eum.