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Covalent Bonding in Actinide Sandwich Molecules Print

Glenn Seaborg was one of the first scientists to recognize that differences in the degree of covalent bonding in lanthanide and actinide compounds could have profound consequences for their unique chemical reactivity and physical properties. Now, researchers working at ALS Beamline 11.0.2 have found evidence for unexpected bonding interactions in two organometallic actinide "sandwich" complexes that have been lightning rods in discussions of covalent bonding since their discovery by Berkeley researchers in the 1960s. Certain organometallics, which have multiple, highly covalent metal–carbon bonds, are vital as industrial or bioinorganic catalysts and as precursors for nanomaterial synthesis. The work at the ALS also provides conclusive evidence for a new form of bonding, proposed in theoretical studies but never observed experimentally, that is strictly a unique capability of lanthanide and actinide elements.


Bonding in the f-Block

Two rows at the bottom of the periodic table stick out like a sore thumb. The metallic elements in these rows are collectively referred to as the "f-block" because their valence (outermost) electrons occupy f-orbitals, the fourth orbital type in the series labeled (for historical reasons) s-p-d-f. When the valence orbitals of two atoms combine to create a covalent bond, the new molecular orbital is designated by the corresponding Greek letters, σ-π-δ-φ.

The extent to which the f-orbitals of actinides (the second of the two rows) mix with orbitals of carbon in various configurations has been the subject of debate. Development of more precise models of the electronic structure in molecules with metal–carbon multiple bonds would be an important conceptual advance, crucial for improvements in organometallic catalysts that are employed across industry for the polymerization of organic molecules.

In this study, Minasian et al. quantitatively probe metal–carbon bonding. Because the thin crystalline samples are encapsulated in silicon nitride, their approach is well suited for work with organometallic molecules that are frequently prone to rapid decomposition on exposure to air or moisture. The results show surprising variability in the amount of metal–carbon covalent bonding with neighboring metals in the periodic table, illustrating the pitfalls associated with using simple models to describe bonding in organometallics.

Upper-left diagram depicts an organometallic actinide sandwich complex, or "actinocene," in which the metal atom (in this case M = Th or U) is centered between two stacked carbon-based organic rings. The remaining diagrams represent the seven possible valence orbital interactions for this complex. Orbitals for the metal atoms are located at the center, and carbon orbitals are shown as eight-membered rings using a standard shorthand notation.

Organometallic chemistry has furnished powerful industrial catalysts, enabled new organic transformations, and is at the heart of functionality in bioinorganic molecules (vitamin B12, for example). Yet in many cases, the models of electronic structure used to describe bonding in organometallics are at frequently at odds with classical coordination chemistry, in that they invoke a covalent bond between the metal and the carbon-based ligands. By combining the synthesis of organometallics with techniques that can probe metal–carbon covalent bonding, researchers can develop fundamental insights that help interpret chemical phenomena across the periodic table.

In recent years, x-ray absorption spectroscopy (XAS) measurements at the ligand K-edge have provided quantitative information regarding covalent bonding and electronic structure in a wide variety of molecules and materials. The presence of covalent mixing is observed as a pre-edge feature by ligand K-edge XAS; it appears only if the vacant metal orbitals contain a component of ligand p-orbital character. Until now, successful application of this technique at the carbon K-edge to study organometallic bonding has been thwarted by weakly penetrating soft x-ray radiation (about 280 eV) that is often susceptible to surface contamination and saturation effects from the samples. Interpreting carbon K-edge XAS results for organometallics is also challenging and requires a comprehensive theoretical effort to deconvolute spectra composed of numerous electronic transitions.

Challenges associated with quantitative carbon K-edge XAS were recently overcome by conducting XAS measurements in transmission using state-of-the-art soft x-ray instrumentation and techniques pioneered at Beamline 11.0.2, the Molecular Environmental Sciences beamline at the ALS. Accurate measurement of pre-edge peak intensities was ensured by preparing thin microcrystalline samples of the air-sensitive and radioactive materials on silicon nitride substrates, which were analyzed using the scanning transmission x-ray microscope (STXM). Samples included actinide sandwich complexes—thorocene [(C8H8)2Th] and uranocene [(C8H8)2U]—"textbook" molecules that have played a central role in the development of theories of covalent bonding in organometallic chemistry. A concomitant hybrid density functional theory (DFT) study provided confidence and credibility for interpretation of the complex spectra.

Left: Normal-contrast images and elemental maps of crystals of thorocene (top) and uranocene (bottom) obtained using STXM from which carbon K-edge XAS were obtained. The images show that the samples were uniform on the micron scale and of appropriate thickness for accurate transmission measurements. Center: Pre-edge region of the carbon K-edge XAS for thorocene (black) and uranocene (green). The carbon 1s → e3u transition is much more intense for thorocene. Taken together with results from hybrid DFT calculations (right), there is strong support for a significant amount of φ covalency in thorocene. The φ-interaction is characterized by six orbital phase changes about the center axis.

Results from the detailed experimental and theoretical study showed two contrasting trends in orbital mixing, suggesting that covalent bonding does not increase uniformly as the actinide series is traversed from left to right on the periodic table. And while most chemical structures are composed of σ and π bonds (involving s and p atomic orbitals), the carbon K-edge XAS spectrum of thorocene represents the first experimental evidence of a measurable φ orbital interaction, corresponding to the overlap of f atomic orbitals.

This work supports many theoretical analyses that predict that the lanthanide and actinide elements are uniquely capable of participating in these unconventional bonding interactions because of the increased size and nodality of their 4f and 5f orbitals. Such discoveries have important implications for the design of actinide-selective extraction processes and may be necessary in efforts to improve on the properties of advanced materials incorporating lanthanide elements.



Research conducted by: S.G. Minasian (Los Alamos National Laboratory and Berkeley Lab); J.M. Keith, E.R. Batista, K.S. Boland, D.L. Clark, S.A. Kozimor, and R.L. Martin (Los Alamos National Laboratory); D.K. Shuh (Berkeley Lab); and T. Tyliszczak (ALS).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

S.G. Minasian, J.M. Keith, E.R. Batista, K.S. Boland, D.L. Clark, S.A. Kozimor, R.L. Martin, D.K. Shuh, and T. Tyliszczak, "New evidence for 5f covalency in actinocenes determined from carbon K-edge XAS and electronic structure theory," Chem. Sci. 5, 351 (2014).

ALS Science Highlight #290


ALSNews Vol. 353