The concept of covalency is essential to descriptions of organometallic bonding, which can consist of formal M�C single, double, and triple bonds. Recent advances have shown that the 1s to np transition intensities measured by ligand K-edge X-ray absorption spectroscopy (XAS) relate to coefficients of orbital mixing. However, it has been difficult to obtain quantitative intensity information at the K-edge for carbon. The advent of the scanning transmission X-ray microscope (STXM) coupled with new sample preparation methods have made such studies a reality. Efforts initiated with a family of organometallic transition metal compounds and have investigated the early actinide metallocenes. The C K-edge spectra of bis-cyclopentadienyltitanium, zirconium, and hafnium dichloride are interpreted with the aid of published Cl K-edge spectra. As expected from group theory, M�C covalency rises from Ti to Hf as the d-orbitals become more diffuse and overlap improves. In contrast, spectra for the heavier thorium and uranium metallocenes show that trends in molecular orbital mixing are driven by changes in the relative degeneracy of the parent atomic orbitals. The evidence for an unusual phi-interaction in thorocene will also be discussed. Finally, results from collaboration with theorists will tie these experimental trends in M�C orbital mixing to changes in 3d, 4d, 5d/4f, and 6d/5f valence orbital occupancies.

Quantifying covalency in metal�carbon bonds in the early actinocenes using carbon K�edge X-ray absorption spectroscopy

248th National Meeting (2014)

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