Development of palladium L-edge X-ray absorption spectroscopy and its application for chloropalladium complexes

X-ray absorption spectroscopy (XAS) is a synchrotron-based experimental technique that provides information about geometric and electronic structures of transition metal complexes. Combination of metal L-edge and ligand K-edge XAS has the potential to define the complete experimental ground state electronic structures for metal complexes with unoccupied d manifolds. We developed a quantitative treatment for Pd L-edge spectroscopy on the basis of the well-established chlorine K-edge XAS for a series of chloropalladium complexes that are pre-catalysts in various organic transformations. We found that Pd–Cl bonds are highly covalent, such as 24 ± 2%, 34 ± 3%, and 48 ± 4% chloride 3p character for each Pd–Cl bond in [PdCl4]2−, [PdCl6]2−, and PdCl2, respectively. Pd(2p → 4d) transition dipole integrals of 20.8 (SSRL)/16.9 (ALS) eV and 14.1 (SSRL)/11.9 (ALS) eV were determined using various combinations of L-edges for Pd(II) and Pd(IV), respectively. Application of metal–ligand covalency and transition dipole integrals were demonstrated for the example of bridging chloride ligands in PdCl2. Our work lays the foundation for extending the quantitative treatment to other catalytically important ligands, such as phosphine, phosphite, olefin, amine, and alkyl in order to correlate the electronic structures of palladium complexes with their catalytic activity.