The bonding of acetylene and ethylene in high-valent and low-valent transition metal compounds

The equilibrium geometries of the transition metal compounds WCl(4)L, WCl(5)L(-) and W(CO)(5)L (L = acetylene, ethylene) are theoretically predicted at the HF and MP2 levels of theory using a relativistic effective core potential for tungsten and valence shell basis sets of DZ + P quality. The W-L dissociation energies are calculated at the CCSD(T) level of theory. The calculated geometries are in very good agreement with experimental values. The W-C-acetylene and W-C-ethylene bond distances of WCl(4)L are much shorter than the bond lengths of W(CO)(5)L. However, the (CO)(5)W-L bond dissociation energies are higher than or comparable in magnitude with the Cl4W-L bond energies. This result can be explained by the different nature of the tungsten-carbon bonds in W(CO)(5)L and WCl(4)L, as revealed by the charge decomposition analysis (CDA) of the compounds. The W-C bonds of the low-valent carbonyl complexes have donor-acceptor character and the bonding can be understood in terms of the Dewar-Chatt-Duncanson model. The tungsten-carbon bonds of WCl(4)L are polar covalent bonds which are formally formed from the triplet states of WCl4 and L. The dissociation energies of WCl(4)L are very low, because the energy which is necessary to promote acetylene and ethylene into the lowest lying triplet state is very high. The CDA results for WCl(5)L(-) suggest that the anions might also be considered as complexes which can be discussed in terms of closed-shell orbital interactions. The Cl5W--L bond energies are rather low, because there is strong repulsion between the occupied orbitals of the fragments. The breakdown of the donor-acceptor interactions into orbital contributions shows that acetylene is a four-electron donor in WCl5(HCCH)(-), while it is a two-electron donor in W(CO)(5)(HCCH). The donation from the out-of-plane CC-bonding pi orbital of acetylene is the reason why WCl5(HCCH)(-) is a stable (isolable) compound. This orbital is not available for ethylene, and thus WCl5(C2H4)(-) is predicted to have a weak tungsten-ethylene bond.