The Helium(I) Photoelectron Spectra and Electronic Structure of (η⁵-Cyclopentadienyl) d⁶ Metal Carbonyls

The He(I) photoelectron spectra of CpMn(CO)₃, CpRe(CO)₃, and CpFe(CO)₂X, where Cp = η⁵-C₅H₅ and X = Cl, Br, I, and CH₃, are presented. The low energy ionization bands (binding energies from 7 to 13 eV) are found to be sensitive to metal and ligand substitution. The ionizations which are associated primarily with the cyclopentadienyl ring π e₁″ levels display a characteristic band envelope. Interestingly, the degeneracy of the ionizations associated predominantly with the metal d levels in octahedral M(CO)₆ complexes and C_4v Mn(CO)₅X complexes is not substantially removed in these lower symmetry CpM(CO)₃ and CpFe(CO)₂X analogues. In the case of CpRe(CO)₃, a distinct spin-orbit splitting of the predominantly metal ionizations is observed. Comparison of the spectra of the CpFe(CO)₂X complexes with the spectra of the corresponding Mn(CO)₅X complexes provides additional information regarding the assignment of the valence ionization bands of the molecules in both of these series. An ab initio calculation on the cyclopentadienide ion and approximate calculations on the transition metal complexes are used to aid in the interpretation of these ionizations. In addition, the details of a method for interpreting the ionizations of a molecule containing an atom with appreciable spin-orbit interaction are presented, and applied to the spectrum of CpRe(CO)₃. The combined knowledge provided by this work and earlier investigations leads to a consistent description of the electronic structure and bonding of d⁶ metal η⁵-cyclopentadienyl carbonyls and indicates that serious errors may result if ionization potentials are interpreted solely on the basis of Koopmans' theorem. The lowered carbonyl force constants in these π-bonded ring complexes (when compared to the corresponding M(CO)₆ and Mn(CO)₅X complexes) are not found to be a result of electron donation from the ring, as previously believed. Instead, the carbonyl ligands and the ring interact with metal orbitals of primarily different symmetry, and the lower carbonyl force constants are a direct result of a decrease in competition for the back-bonding electrons from the metal orbitals of appropriate symmetry.