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

The equilibrium geometries of the transition metal compounds WCl₄L, WCl₅L and W(CO)₅L (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 WCl₄L are much shorter than the bond lengths of W(CO)₅L. However, the (CO)₅W - 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)₅L and WCl₄L, 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 WCl₄L are polar covalent bonds which are formally formed from the triplet states of WCl₄ and L. The dissociation energies of WCl₄L 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 WCl₅L^- 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 π 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.