Energy partitioning analysis of the bonding in L2TM-C2H2 and L2TM-C2H4 (TM = Ni, Pd, Pt; L2 = (PH3)2, (PMe3)2, H2PCH2PH2, H2P(CH2)2PH2

The equilibrium geometries and bond dissociation energies of the complexes L2TM-C2H2 and L2TM-C2H4 (TM = Ni, Pd, Pt) with the monodentate ligands L-2 = (PH3)(2), (PMe3)(2) and the bidentate ligands L-2 = eta(2)-diphosphinomethane (dpm), eta(2)-diphosphinoethane (dpe) have been calculated using gradient-corrected DFT methods. The nature of the bonding interactions between the metal and the 7 ligands ethene and ethyne was investigated with an energy partitioning analysis (EPA). The ethene and ethyne ligands are more strongly bonded to the metal when L-2 = dpm, dpe. The EPA results reveal that the reason for the stronger bonds of (dpm)TM-C2Hx and (dpe)TM-C2Hx is the smaller preparation energy of (dpm)TM and (dpe)TM that is necessary to deform the metal fragments from the equilibrium geometry to the geometry in the complex. The L2TM-C2Hx interaction energies between the fragments with a frozen geometry do not significantly vary when L-2 consists of a bidentate or two monodentate ligands. The EPA shows also that the nature of the L2TM-C2Hx bonding does not change a lot when L2 = (PH3)(2), (PMe3)(2) or when L-2 = dpm, dpe. The metal-carbon bonds always have a higher electrostatic (54.1-62.3%) than covalent (37.7-45.9%) character. The covalent bonding in the ethyne and ethene complexes comes mainly from the TM-->C2Hx in-plane pi back-donation, while the relative contribution of the TM <-- C2Hx sigma donation is much less. The contributions of the out-of-plane a(2)(delta) and b(1)(pi(perpendicular to)) orbital interactions are very small even for the ethyne complexes. The bonding analysis suggests that the ethyne ligand in the complexes (PH3)(2)TM-C2H2 and (PMe3)(2)TM-C2H2 should be considered as a two-electron donor and not a four-electron donor.