Electronic structure of the ferricenium cation

Abstract

E.S.R. spectra of cations of substituted ferrocenes have been measured at 20° and 77°k. The g values are highly anisotropic, with g _¶ = 4·35 and g⊥=1·26 for Fe(C₅H₅)₂ ⁺. Owing to a fast spin-lattice relaxation the E.S.R. signals are much broader and less intense at 77° than at 20°k. A theoretical analysis shows that Fe(C₅H₅)₂ ⁺ has the orbitally degenerate ² E _2g [(e _2g )³(a _{1g ²} ] ground-state configuration. The combined action of the spin-orbit interaction H _so and a low-symmetry perturbation H ₁ splits the fourfold degenerate level into two Kramers doublets. Since H _so and H ₁ are of the same order of magnitude, the orbital momentum contribution to the g values is only partly quenched and the g values deviate strongly from the free-electron value. The predicted splitting of the Kramers doublets is small and this explains the fast relaxation. The e _2g molecular orbitals are essentially localized on the iron atom. Substitution influences primarily the symmetry of the complex; it has no great influence on the mixing of the iron 3d ₂ and ligand e _2g π orbitals. g values and line widths of the E.S.R. signals indicate that in Fe(C₅H₅)₂ ⁺ the perturbation of the axial symmetry has an external origin. In the substituted complexes internal perturbations are also present. By a combination of E.S.R., visible absorption and photoelectron spectra the order and character of the highest filled and lowest empty orbitals in ferrocene have been found to be: