ELECTRONIC-STRUCTURE PERTURBATIONS OF SUBSTITUTED RUTHENOCENES - THE FIRST PHOTOELECTRON-SPECTRA OF PERCHLOROCYCLOPENTADIENYL AND PERFLUOROCYCLOPENTADIENYL COMPLEXES

The electronic structure perturbations caused by cyclopentadienyl subs titutions in the series of complexes (eta(5)-C5H5)(2)Ru, (eta(5)-C5Me5 )(eta(5)-C5H5)Ru, (eta(5)-C5Me5)(2)Ru, (eta(5)-C5Me5)(eta(5)-C-5-Cl-5) Ru, and (eta(5)-C5Me5)(eta(5)-C5F5)Ru are measured by gas-phase photoe lectron spectroscopy. The shifts of the valence metal-and cyclopentadi enyl-based ionizations give an indication of the overall electronic ef fects of methyl and halogen substitutions on the cyclopentadienyl ring s. The halogen substituent interaction is an admixture of inductive si gma-electron-withdrawing and filled-filled pi-electron-overlap effects , which act in opposite directions. The it-overlap interaction is rela tively weak in the case of chlorine substitution for hydrogen, and the combined sigma and pi interactions give rise to an overall withdrawal of electron density from the metal center and increase in the metal d -based ionization energies. Fluorine substituents on cyclopentadienyl make the ring only slightly more electron withdrawing than eta(5)-C5Cl 5, despite the much greater electronegativity of fluorine compared to chlorine. The electron withdrawing ability of eta(5)-C5F5 is tempered by the greater filled-filled interaction of the fluorine p(pi) orbital s with the cyclopentadienyl p(pi) orbitals, which lessens the stabiliz ation of these orbitals and the withdrawal of electron density from th e metal. It is interesting that in each case the metal d-based ionizat ions are stabilized more than the cyclopentadienyl pi-based ionization s with halogen substitution for hydrogen, such that these ionizations begin to merge in the lowest ionization energy band.