Strong double excitation and open-shell features in the near-edge x-ray absorption fine structure spectroscopy of ferrocene and ferrocenium compounds.

We have recorded the iron 2p and carbon 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of decamethyl ferrocene (Fe 3d(6)) and ferrocenium hexafluorophosphate (Fe 3d(5)), two low spin compounds with closed- and open-shell electron configurations, respectively. These species have a similar covalent bonding environment but a different electron configuration, allowing us to examine the role of ligand-metal interactions and electron correlation responsible for their NEXAFS spectra. We have interpreted these spectra with the aid of ab initio configuration interaction (CI) calculations. The CI calculations indicate that double excitation is essential to interpret the Fe 2p NEXAFS spectra of not only the open-shell ferrocenium ion but also the closed-shell ferrocene species, even though the ground states of both species are well described within the Hartree-Fock single-configuration approximation.