Energy Partitioning Analysis of the Bonding in Ethylene and Acetylene Complexes of Group 6, 8, and 11 Metals: (CO)₅TM-C₂H _x and Cl₄TM-C₂H_x (TM = Cr, Mo, W), (CO)₄TM-C₂H_x (TM = Fe, Ru, Os), and TM ⁺-C₂H_x (TM = Cu, Ag, Au)

Quantum chemical calculations at the DFT level have been carried out for the title compounds. The equilibrium geometries and bond dissociation energies are reported. The nature of the bonding between the metal and the π ligands ethylene and acetylene has been investigated by means of an energy partitioning analysis. The nature of the metal-ligand interactions is not very different from each other in the donor-acceptor complexes (CO)₅TM-C ₂H_x (TM = Cr, Mo, W), (CO)₄TM-C ₂H_x (TM = Fe, Ru, Os), and TM⁺-C ₂H_x (TM = Cu, Ag, Au). The metal-C₂H _x bonds have a slightly more electrostatic than covalent character. The covalent bonding comes mainly from the metal ← ligand σ donation and the metal → ligand π∥ in-plane back-donation. The contributions from the out-of-plane π⊥ and δ orbitals are negligible. The main difference of the bonding interactions in the metallacyclic compounds Cl ₄TM-C₂H_x (TM = Cr, Mo, W) is that they are clearly more covalent than electrostatic. The covalent interactions come also mainly from a₁(σ) and b₂(π∥) interactions. The a₂(δ) orbital interactions are negligible but the interactions of the out-of-plane π orbitals in the acetylene complexes CUTM-C₂H₂ contribute ∼ 11% to the total orbital term.