Studies of Substituent Effects on Silver( I)-olefin Complexation Using Ab lnitio Molecular Orbital Calculations

Abstract

The ab initio molecular orbital are performed to study the effects of substituents on the complexation between silver(1) and ethylene derivatives. It is found that substituents with electron-withdrawing ability attribute to decreases in the energies of the olefinic ∏- and ∏*-orbitals. On the contrary, an increase of the orbital energies is observed as a hydrogen atom in ethylene is replaced by an electron-donating substituent. The changes in ∏- and ∏*-orbital energies alters the energy difference between the appropriate orbital pairs involving in the bonding, and consequently affects the stability of the complex. The importance of the o-bond to the formation of a stable complex is underlined by a smaller energy gap between the silver(1) 5s- and the olefinic ∏-orbitals relative to that of the silver(1) 4d- and the olefinic ∏*-orbitals. Binding energies of the complexes are also calculated, and found to relate reasonably well to electron availability from the olefinic ∏-orbital. The intimate correspondence of the correlations between binding energy and the reciprocation of the energy difference between the olefinic n-orbital and the silver(1) 5s-orbital with respect to Hammett substituent constant is a clear indication that substituent has negligible effect on the overlap extent. The relative stability of the silver(1)-olefin complexes is found to relate to the mulliken charge of the complexed silver and the increase in carbon-carbon double bond distance of olefin after the complexation.