An Example of the Influence of the Ligand Field on the Electron Localisation: The Mixed Valence Ferrocene-Ferricinium

We performed MO calculations for the ferrocene-ferricinium cation, in the frame of the PKS adiabatic model, i.e. as a function of an assymmetric distortion of the molecule. Computed quadrupole splittings and energy schemes unambigously showed that localisation requires a HOMO made of dxy weakly coupled orbitals; on the contrary, a HOMO made of dx 2-y 2 strongly coupled orbitals, leads to a delocalised ground state. The actual type of the HOMO is a result of low-symmetry ligand fields acting on each sub-unit.