Covalency in Americium(III) Hexachloride

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 difficultly 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 f -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–Cl interaction within AmCl 6 3– . We observed both 5 f - and 6 d -orbital mixing with the Cl-3 p orbitals; however, contributions from the 6 d -orbitals were more substantial. Comparisons with the isoelectronic EuCl 6 3– indicated that the amount of Cl 3 p -mixing with Eu III 5d-orbitals was similar to that observed with the Am III 6 d -orbitals. Meanwhile, the results confirmed Seaborg’s 1954 hypothesis that Am III 5 f- orbital covalency was more substantial than 4 f -orbital mixing for Eu III .