Phosphine versus Carbene Metal Interactions: Bond Energies

A variety of different ground-state structures of carbene and phosphine groups 1 and 2 cationic, group 11 cationic, and group 10 neutral complexes were studied using density functional theory (DFT) and correlated molecular orbital theory (CCSD(T)) methods. Geometries of complexes with phosphines were studied and compared to available experimental data. Among the three analyzed phosphine ligands, PH₃, PMe₃, and PPh₃, PH₃ was found to have noticeably smaller ligand binding energies (LBEs, ΔH_298 K). PPh₃ has the greatest LBEs with group 2 dications. The difference in LBEs for PMe₃ and PPh₃ in complexes with group 1 monocations and transition-metal (TM) complexes was significantly less pronounced. The stability and reactivity of phosphine complexes were analyzed and compared with those of previously studied N-heterocyclic carbenes (NHC). PH₃ has smaller LBEs compared to NHC carbenes. The lower LBEs correlate with the hardness for M(11)⁺ complexes and correlate with both the hardness and ionic radii for the M(1)⁺ and M(2)²⁺ complexes. The presence of additional PH₃ substituents on the metal center makes the LBE smaller compared to their unsubstituted or less substituted analogs. The presence of NH₃ in a structure causes a smaller effect on binding, and, except for carbene-PtNH₃, an increase in LBE was observed. Composite-correlated molecular orbital theory (G3MP2) was used to predict the LBE of various Lewis acidic ligands with PH₃ and NHCs to contrast their binding behavior. Binding either phosphine or carbene to metal diamine complexes caused ligand exchange and transfer of NH₃ to an outer coordination sphere.