Combined X-Ray Emission Spectroscopy at Phosphorus and Nickel: Detecting Subtle Changes in Catalyst Electronic Structure at High Resolution.

Valence-to-core X-ray emission spectroscopy (VtC XES) is widely used to characterize valence electronic structure, especially of transition metal systems in homogeneous and bioinorganic catalysis. Although metal K-edge VtC XES has proved useful, its observable information content is limited by the large lifetime broadening of the metal 1s core hole, and its practical application is limited by small VtC emission probability and thus low count rates. Ligand VtC XES in transition metal complexes, though largely unexplored, offers a higher resolution and potential for broad applications in catalysis research. Here, P VtC XES is introduced for catalysts with phosphine ligands, perhaps the most important class of spectator ligands in homogeneous catalysis. P VtC XES is sensitive to subtle changes in electronic structure, with difference spectra that are well-reproduced by density functional theory (DFT) calculations, indicating that DFT can not only provide insight into the physical origins of spectral features but can also facilitate the identification of unknown species. Comparison to Ni VtC XES, as well as previously published X-ray absorption data, establishes the high resolution and complementary benefits of the technique. The potential of P VtC XES as a metal- and spin-agnostic tool for experimentally assessing electronic structure and mechanisms in phosphine-coordinated catalysts is highlighted.