Heavy Element Chemistry
Jaqueline L. Kiplinger - Capability Leader
The actinide series marks the emergence of 5f electrons in the valence shell. The involvement of 5f electrons in actinide bonding, from molecules to materials, has been the central and integrating focus of actinide chemistry and physics over the past 30 years. In the pure elements, those left of Pu have delocalized (bonding) electrons, while elements to the right of Pu are localized (non-bonding). Plutonium can exhibit both behaviors, depending on its bonding environments, oxidation state, etc. While the 5f electrons clearly participate in the extended band structure of light actinide metals and some alloys, whether 5f electrons participate significantly in covalent bonding in molecular compounds is remains a fascinating open question.
Chemists use a slightly different language to describe the localized and delocalized endpoints in this spectrum of behavior - the localized electrons give rise to ionic behavior while delocalized electrons are those involved in covalent bonding. A common viewpoint from the literature is that f element complexes are essentially ionic in nature, based on the premise that f orbitals are core-like and unable to overlap effectively with ligand orbitals. This premise is arguably well-justified for 4f series complexes. In classical lanthanide complexes, the coordination numbers and geometries are often poorly defined in solution, as the metal exerts little or no covalent control over the spatial arrangement of the ligands. This is also what is observed for the heavier actinide elements beyond plutonium. Whether this lack of covalency extends to the light actinide series has been widely debated. The 5f orbitals of the lighter actinides are substantially extended from the core, and there are certain classes of compounds where the 5f orbitals have been shown to play a significant role in covalent metal-ligand bonding. Thus, this shared interest in unraveling the subtleties of 5f electron behavior from molecules to materials is the single greatest integrating element between the chemistry, physics, and materials science communities at Los Alamos and internationally.
Organometallic chemistry has proven to be an ideal foundation to investigate actinide-ligand covalent interactions because of the exquisite synthetic control it provides and the breadth of structural motifs it enables. Our team maintains collaborations across the Lab and uses a variety of state-of-the-art characterization techniques in our studies including high-field multinuclear NMR, UV-visible-NIR, X-ray absorption, emission, infrared, resonance Raman spectroscopies, magnetic susceptibility measurements, and X-ray diffraction. We are interested in developing a fundamental understanding of the nature of chemical bonding and the relative roles of valence 5f and 6d orbitals through reaction chemistry and to explore manifestations of covalent metal-ligand bonding in complexes of uranium and the other light actinides over a broad range of ligand sets and structure types through the combination of synthetic organometallic chemistry, electronic structural characterization, and density functional theory. Under the unifying concept of deriving a fundamental understanding of the factors that give rise to covalent interactions in f-element systems our team is involved in the following project areas:
I. Actinide-Ligand Multiple Bonds
National High Magnetic Field Laboratory/NHMFL
Low Energy Spectroscopy