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

Abstract 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 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 Cl 4 W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 binding 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 WCl 4 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 Cl 5 W − 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 WCl 5 (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 WCl 5 (HCCH) − is a stable (isolable) compound. This orbital is not available for ethylene, and thus WCl 5 (C 2 H 4 ) is predicted to have a weak tungsten-ethylene bond.

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