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 5f-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 AmCl63–. We observed both 5f- and 6d-orbital
mixing with the Cl-3p orbitals; however, contributions
from the 6d-orbitals were more substantial. Comparisons
with the isoelectronic EuCl63– indicated
that the amount of Cl 3p-mixing with EuIII 5d-orbitals was similar to that observed with the AmIII 6d-orbitals. Meanwhile, the results confirmed Seaborg’s
1954 hypothesis that AmIII 5f-orbital
covalency was more substantial than 4f-orbital mixing
for EuIII.