Probing the molecular orbitals and charge redistribution in organometallic (PP)Pd(XX) complexes. A Pd K-edge XANES study.

Pd K-edge X-ray absorption near-edge spectroscopy (XANES) is used to probe the unoccupied molecular orbitals in bidentate diphosphine Pd complexes. Complexes containing a series of bidentate diphosphine ligands (PP) are examined to study the effect of the ligand bite angle on the charge redistribution in these complexes. Different coordinating moieties (XX) have been used to induce a range of Pd oxidation states. A full interpretation of the Pd K-edge XANES data is presented. The negative second derivative of these XANES data provides direct information on the energy and electronic distribution of the different unoccupied molecular orbitals probed. The charge redistributions within the complexes, as reflected in the effective Pd oxidation state, are indicated by both the intensity of the first edge feature, the "Pd d peak", and the energy of the second edge feature, the "Pd p peak", which can be easily observed in the negative second derivative of the XANES data. Additionally, the changing covalent interaction between the Pd and coordinated moieties via the Pd p orbitals is reflected directly in the energy splitting of the "Pd p" peak. Thus, investigation of these (PP)Pd(XX) complexes, some used as catalysts in organic synthesis, with XANES spectroscopy provides new essential information on their electronic properties. Further, the XANES analysis techniques described in this paper can be applied to investigate the unoccupied molecular orbitals and charge redistributions within a wide range of samples.