Development of palladium L-edge x-ray absorption spectroscopy and its application on chloro palladium complexes

Abstract

X-ray Absorption Spectroscopy (XAS) is a synchrotron-based experimental technique that can provide information about geometric and electronic structures of transition metal complexes with unoccupied d orbitals. Combination of metal L-edge and ligand K-edge XAS has the potential to define the experimental ground state electronic structure. We developed a quantitative treatment for Pd L-edge spectroscopy based on the already established Cl K-edge XAS for a series of chloro palladium complexes, which are precatalysts in numerous organic transformations. We found that Pd-Cl bonds are highly covalent (23% per Cl in [PdCl₄]²⁻, 34% per Cl in [PdCl₆]²⁻, and 46% in PdCl₂). Dipole integrals for Pd(2p..4d) transitions of 42 eV for Pd(II) and 48 eV for Pd(IV) LIII-edges and 39 eV and 35 eV, respectively, at these Pd oxidation states for the LII-edges were determined. Application of the metal-ligand covalencies and transition dipole integrals by describing the ground state bonding in PdCl₂ with bridging Cl ligands was demonstrated. In future studies, a similar approach will be utilized for palladium phosphine, allyl, olefin complexes in order to define their experimental electronic structure and correlate this with their observed reactivity.