Energy Partitioning Analysis of the Bonding in L2TM−C2H2and L2TM−C2H4(TM = Ni, Pd, Pt; L2= (PH3)2, (PMe3)2, H2PCH2PH2, H2P(CH2)2PH2)

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The equilibrium geometries and bond dissociation energies of the complexes L2TM−C2H2 and L2TM−C2H4 (TM = Ni, Pd, Pt) with the monodentate ligands L2 = (PH3)2, (PMe3)2 and the bidentate ligands L2 = η2-diphosphinomethane (dpm), η2-diphosphinoethane (dpe) have been calculated using gradient-corrected DFT methods. The nature of the bonding interactions between the metal and the π 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 L2 = 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 L2 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 L2 = 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 π back-donation, while the relative contribution of the TM←C2Hx σ donation is much less. The contributions of the out-of-plane a2(δ) and b1(π⊥) orbital interactions are very small even for the ethyne complexes. The bonding analysis suggests that the ethyne ligand in the complexes (PH3)2TM−C2H2 and (PMe3)2TM−C2H2 should be considered as a two-electron donor and not a four-electron donor.