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)_<i>n</i>PPh₂, where <i>n</i> = 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)₂Cl₂ (<b>1</b>) to square planar in Ni­(dppe)­Cl₂ (<b>2</b>) more than doubles the intensity of pre-edge features assigned to Ni–P and Ni–Cl 1s → σ* transitions. By way of comparison, varying the diphosphine in Pd­(dppm)­Cl₂ (<b>4</b>), Pd­(dppp)­Cl₂ (<b>6</b>), and Pd­(dppb)­Cl₂ (<b>7</b>) yielded Pd–P 1s → σ* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd­(dppe)­Cl₂ (<b>5</b>). A similar observation was made when comparing Ni­(dppe)­Cl₂ (<b>2</b>) to Ni­(dppp)­Cl₂ (<b>3</b>), 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 <i>trans</i>-Pd­(PPh₃)₂Cl₂ (<b>8</b>) for comparison to the cis diphosphine complex Pd­(dppe)­Cl₂ (<b>5</b>). Consistent with phosphine’s stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd–P 1s → σ* pre-edge feature and a complementary 34% increase in Pd–Cl 1s → σ* feature was observed for <b>8</b> (trans) compared to <b>5</b> (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