Diphosphines are commonly used ligands in transition metal catalysis because their substituents and structures can be modified to optimize catalyst performance. For example, variations to bite angle (i.e. the P-M-P angle) have been shown to affect catalysis rates, but it is less clear how bite angle affects bonding between the metal and phosphorus atoms. To investigate this relationship, we used ligand K-edge X-ray absorption spectroscopy (XAS) to determine how bite angle variations affect covalency in metal-ligand bonds. This technique probes transitions between 1s electrons localized on the ligand donor atom and the antibonding molecular orbitals of metal-donor atom bonds. Peak positions in the spectra represent the energies of electronic transitions, while peak intensities relate to the amount of 3p ligand character mixing in the MOs. P and Cl K-edge XAS data were collected on a series of NiCl2 and PdCl2 complexes to investigate differences in M-P bonding and M-Cl bonding as ligands were systematically changed between PPh3, dppm, dppe, dppp, and dppb. DFT and TDDFT calculations were performed to corroborate our experimental findings, as well as study the effects of bite angle versus twist angle. Results from both P and Cl K-edge data showed the expected differences between tetrahedral and square-planar geometries, as well as differences caused by changing the metal from a first row (Ni) to second row transition metal (Pd). P and Cl K-edge data of the Pd chelating diphosphine complexes show a small systematic shift in the position of the spectral features, but the intensities are closely matched except in the case of Pd(dppe)Cl2.

Investigating the stereoelectronic consequences of diphosphine ligands on M-P and M-Cl covalency in late transition metal complexes

249th National Meeting (2015)

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