Covalency in Americium(III) Hexachloride

Abstract

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 AmCl. 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 EuCl indicated that the amount of Cl 3p-mixing with Eu 5d-orbitals was similar to that observed with the Am 6d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that Am 5f-orbital covalency was more substantial than 4f-orbital mixing for Eu.

Solution and Solid‐State Ligand K‐Edge XAS Studies of PdCl2 Diphosphine Complexes with Phenyl and Cyclohexyl Substituents

“…To evaluate the possible changes in Pd–P covalency in solution and the solid‐state, we performed dispersion‐corrected DFT and TDDFT calculations on 1 – 3 with a polarizable continuum model to simulate the CH 2 Cl 2 solution. The B3LYP functional was again selected because it has been shown to be remarkably accurate in the simulation of experimental XAS spectra via time‐dependent density functional theory (TDDFT) , , , , , , . For consistency, TDDFT calculations for 1 – 3 from our previous study were recalculated with dispersion corrections for comparison to the dispersion‐corrected solution calculations presented here.…”

“…Ligand K‐edge X‐ray absorption spectroscopy (XAS) is a powerful method for experimentally quantifying differences in covalent metal–ligand bonding and electronic structure in transition metal, lanthanide, and actinide coordination compounds . Ligand K‐edge XAS – where ligand refers to the atom bound to the metal – uses synchrotron‐generated X‐rays to excite 1s electrons localized on the ligand into unoccupied molecular orbitals (MOs) containing ligand p character.…”

Energy-Degeneracy-Driven Covalency in Actinide Bonding

“…For example, there is a body of evidence that demonstrates -in certain systemssubstantial 5f-and 6d-covalency exists. (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) These data are juxtaposed with numerous studies suggesting that An-ligand bonding is primarily ionic. (21) The later conclusions are fueled by several observables that include (but are not limited to) An-ligand bond distances being routinely predicted using actinide ionic radii and an appreciable body of spectroscopic data that shows the 5f  5f optical transitions are marginally impacted by the ligand field.…”

“…To directly address these challenges in actinide bonding, we have characterized covalency for Pu IV and the other early actinides (Th IV , U IV , Np IV ) within an Oh-AnCl6 2 framework using a combination of ligand K-edge X-ray absorption spectroscopy (XAS) (22) and spin-orbit density functional theory (DFT) transition dipole-moment calculations. (9,(23)(24)(25) This duo has recently emerged as one of the most powerful approaches for characterizing metal-ligand covalency. For example, ligand K-edge XAS probes dipole-allowed bound-state transitions from the ligand 1sorbitals to molecular orbitals that contain ligand p-character resulting from metal-ligand orbital mixing.…”

Transuranic Computational Chemistry

“…Indeed, the Los Alamos group has very recently reported their first application of ligand K-edge XAS to at ransuranic target;A mCl 6 3À . [39] Given the discussions presented in this review concerning the radial extent of the 5f orbitals as the actinides eries is traversed, it is not surprising to learn that this latest study finds significantly more Am 6dbased covalency than 5f.…”