Electronic structure perturbations of substituted ruthenocenes: The first photoelectron spectra of perchloro- and perfluorocyclopentadienyl complexes

The electronic structure perturbations caused by cyclopentadienyl substitutions in the series of complexes (η⁵-C₅H₅)₂Ru, (η⁵-C₅Me₅)(η⁵-C ₅H₅)Ru, (η⁵-C₅Me₅)₂Ru, (η⁵-C₅Me₅)(η⁵-C ₅-Cl₅)Ru, and (η⁵-C₅Me₅)(η⁵-C ₅F₅)Ru are measured by gas-phase photoelectron spectroscopy. The shifts of the valence metal- and cyclopentadienyl-based ionizations give an indication of the overall electronic effects of methyl and halogen substitutions on the cyclopentadienyl rings. The halogen substituent interaction is an admixture of inductive σ-electron-withdrawing and filled-filled π-electron-overlap effects, which act in opposite directions. The π-overlap interaction is relatively weak in the case of chlorine substitution for hydrogen, and the combined σand π 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 η⁵-C₅Cl₅, despite the much greater electronegativity of fluorine compared to chlorine. The electron withdrawing ability of η⁵-C₅F₅ is tempered by the greater filled-filled interaction of the fluorine p_π orbitale with the cyclopentadienyl p_π orbitals, which lessens the stabilization of these orbitals and the withdrawal of electron density from the metal. It is interesting that in each case the metal d-based ionizations are stabilized more than the cyclopentadienyl π-based ionizations with halogen substitution for hydrogen, such that these ionizations begin to merge in the lowest ionization energy band.