Electronic structure and reactivity of pentamethylcyclopentadienyl complexes of cobalt, rhodium, and iridium : He-I and He-II photoelectron spectroscopic investigation

Abstract

The He-I and He-II photoelectron spectra of [Co(η-C₅Me₅)(CO)₂], [Rh(η-C₅Me₅)(CO)₂], [Ir(η-C₅Me₅)(CO)₂], [Ir(η-C₅Me₅) Me₂(Me₂SO)], and [Ir(η-C₅Me₅) Me₄] are reported and assigned on the basis of simple molecular orbital schemes. The highest occupied molecular orbital of the dicarbonyls is deduced to have more ring-carbon character in the case of the rhodium and iridium compounds than in the case of the cobalt compound. This is correlated with their differing behaviour on one-electron oxidation and their relative ease of CO substitution by two-electron donor ligands. For the series of iridium compounds the first ionization energy increases only very slightly with oxidation state, whereas the average d-orbital ionization energy is greatest for the iridium(I) compound. The ability of the Ir(η-C₅Me₅) moiety to exist in a wide variety of oxidation states is understood in terms of the electron-releasing ability of the pentamethylcyclopentadienyl ligand which enables the metal d orbitals to maintain suitable energy and overlap for covalent bond formation.