The Photoelectron-spectrum of Hexacarbonyl-mu-di-tert-butylacetylene-di-iron - A Joint Experimental and Theoretical-study

The He I and He II photoelectron spectra of [Fe2(CO)6(ButC2But)] are reported and assigned on the basis of ab initio self-consistent field (SCF) and configuration interaction (Cl) calculations carried out on the model system [Fe2(CO)6(HC2H)] assuming a perfect Cs symmetry. The calculations at the SCF level lead to the possibility of a triplet ground state for the model system. However, But substitution and observed deviations from Cs symmetry, along with the pair correlation energy not accounted for in the present work, are expected to reverse this trend. The ionization energies have therefore been computed assuming a closed-shell ground state for the neutral molecule. The highest occupied molecular orbital can be described in terms of a backdonation interaction from both metal atoms to a carbonyl ligand displaying some semibridging character. The Cl calculations, carried out on the ionized states using the molecular orbital set of the neutral molecule were expected to account for both the correlation effects (through two-electron excitations) and the relaxation effects (through one-electron excitations). These relaxation effects were, however, not completely accounted for, especially for localized d orbitals. The origin of the five bands reported between 7 and 12 eV can, however, be assigned with good confidence from the joint interpretation of the experimental and theoretical results.