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 pi 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)(5)TM-C2Hx (TM = Cr, Mo, W), (CO)(4)TM-C2Hx (TM = Fe, Ru, Os), and TM+-C2Hx (TM = Cu, Ag, An). The metal-C2Hx bonds have a slightly more electrostatic than covalent character. The covalent bonding comes mainly from the metal <-- ligand sigma donation and the metal --> ligand pi(ll) in-plane back-donation. The contributions from the out-of-plane pi(perpendicular to) and delta orbitals are negligible. The main difference of the bonding interactions in the metallacyclic compounds Cl4TM-C2Hx (TM = Cr, Mo, W) is that they are clearly more covalent than electrostatic. The covalent interactions come also mainly from a(1)(sigma) and b(2)(pi(ll)) interactions. The a(2)(delta) orbital interactions are negligible but the interactions of the out-of-plane pi orbitals in the acetylene complexes Cl4TM-C2H2 contribute similar to11% to the total orbital term.