Impact of Coordination Geometry, Bite Angle, and Trans Influence on Metal-Ligand Covalency in Phenyl-Substituted Phosphine Complexes of Ni and Pd

Despite the long-standing use of phosphine and diphosphine ligands in coordination chemistry and catalysis, questions remain as to their effects on metal-ligand bonding in transition metal complexes. Here we report ligand K-edge XAS, DFT, and TDDFT studies aimed at quantifying the impact of coordination geometry, diphosphine bite angle, and phosphine trans influence on covalency in M-P and M-Cl bonds. A series of four-coordinate NiCl2 and PdCl2 complexes containing PPh3 or Ph2P(CH2)(n)PPh2, where n = 1 (dppm), 2 (dppe), 3 (dppp), and 4 (dppb), was analyzed. The XAS data revealed that changing the coordination geometry from tetrahedral in Ni(PPh3)(2)Cl-2 (I) to square planar in Ni(dppe)Cl-2 (2) more than doubles the intensity of pre-edge features assigned to Ni-P and Ni-Cl 1s -> sigma* transitions. By way of comparison, varying the diphosphine in Pd(dppm)Cl-2 (4), Pd(dppp)Cl-2 (6), and Pd(dppb)Cl-2 (7) yielded Pd-P 1s -> sigma* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd(dppe)Cl-2 (5). A similar observation was made when comparing Ni(dppe)Cl-2 (2) to Ni(dppp)Cl-2 (3), and DFT and TDDFT calculations corroborated XAS results obtained for both series. Comparison of the spectroscopic and theoretical results to the diphosphine structures revealed that changes in M-P covalency were not correlated to changes in bite angles or coordination geometry. As a final measure, P and Cl K-edge XAS data were collected on trans-Pd(PPh3)(2)Cl-2 (8) for comparison to the cis diphosphine complex Pd(dppe)Cl-2 (5). Consistent with phosphine's stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd-P 1s -> sigma* pre-edge feature and a complementary 34% increase in Pd-Cl 1s -> sigma* feature was observed for 8 (trans) compared to 5 (cis). Overall, the results reveal how coordination geometry, ligand arrangement, and diphosphine structure affect covalent metal-phosphorus and metal-chloride bonding in these late transition metal complexes.