A theoretical study of the electronic structure of ferrocene and ferricinium: Application to Mössbauer isomer shifts, ionization potentials, and conformation

Abstract

Self‐consistent field wavefunctions have been obtained for ferrocene and several low‐lying states of the ferricinium ion using extended basis sets of contracted Gaussian functions. In agreement with the near minimal basis set results of Coutière et al., the electronic structure is found to change very considerably when ferricinium is formed by removing an electron from an Fe d molecular orbital of ferrocene. The ionicity of Fe, as determined by a Mulliken gross population analysis, is found to be +1.47 for the ion and +1.39 for ferrocene. The difference is less than 0.1 electrons, while a value close to one would have been expected. The major contribution to the change in structure comes from a greatly increased covalency of the ligand orbitals of e_1g symmetry in the ion. The change is shown to be responsible for the small differences found between the isomer shifts, and thus electron densities at the Fe nucleus, in ferrocene and ferricinium salts. Direct contributions from Fe 4s electrons are found to account for about half of the 0.93a⁻³₀ difference in the electron density. A value of the change in nuclear radius, δR/R=−4.4×10⁻⁴, is estimated from the computed densities. Computed values of the six lowest ionization potentials of ferrocene are compared with photoelectron spectra, and symmetry assignments of the ionic levels are made. The average error of the computed ionization potentials is 0.5 eV, giving confidence to these assignments. For the states considered, wavefunctions have been obtained for both the eclipsed and staggered conformations of the C₅H₅ rings. Both absolute and relative total energies are found to be essentially the same for either conformation.