Advancing understanding of metal–light atom (C, N, and O) electronic structure and bonding is of widespread interest because these interactions control the physics and chemistry of many important technological processes. This is especially true for systems with σ and π M–C bonds, as demonstrated in industry through polymerization of organic molecules and in nature through catalysis in organometallic reactions. Among the few experimental methods that can be used to quantify metal-ligand orbital mixing, ligand K-edge X-ray absorption spectroscopy (XAS) stands out as unique. Seminal work by Solomon, Hedman, Hodgon, and coworkers has established ligand K-edge XAS as a powerful technique for quantifying covalency for M–Cl and M–S bonds. The method probes bound state transitions between core ligand 1s and unoccupied molecular orbitals, which only have intensity if the empty acceptor orbitals contain ligand p character. In principle, C K-edge XAS can also be used to quantify M–C orbital mixing because the underlying physics governing Cl and S K-edge XAS also holds for carbon. However, the C K-edge (ca. 280 eV) is much lower in energy than that for Cl and S (&gt;2400 eV). Thus, acquiring quantitative C K-edge XAS data are complicated by weakly penetrating incident radiation that is potentially susceptible to surface contamination and saturation effects from the samples. This presentation will describe our efforts to overcome these technical challenges and document recent measurements made on some organometallic systems, like (C5R5)2MX2. Results are presented in the context of previously-published Cl K-edge XAS and DFT analyses of the same (C5H5)2MCl2 metallocenes, which provides a consistent bonding picture from the perspective of both the ancillary C5H51- and reactive Cl1- ligands.

Evaluating metal–carbon orbital mixing in metallocene dichlorides

249th National Meeting (2015)

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