Mechanism of spin delocalization and nature of the ground state in ferricenium cations

Isotropic nuclear magnetic resonance shifts and linewidths for [Fe(C5H5)(2)]PF6 and [Fe(C5H4CH3)(2)]PF6 have been measured at different temperatures between 202 and 324 degrees K. Room temperature solution magnetic moments for various ferricenium salts were also measured. Theoretical expressions for magnetic susceptibilities and ESR g values are deduced which, unlike previous calculations, do not involve any a priori assumptions regarding the separation between E-2(2g) and (2)A(1g) Levels. These expressions. along with the expressions of Kurland and McCarvey, for dipolar and contact shifts are used to provide a consistent interpretation of the existing ESR g values and magnetic susceptibilities as welt as our NMR and magnetic moment results. It is found that the dipolar term contributes about 5596 to the observed resonance shift for the methyl protons but only about 25% to the ring proton shifts. The calculated values of the coupling constants [A(H) = 0.168 +/- 0.005 gauss for Fe(C5H5)(2)(+) and A(H) = 0.161 +/- 0.015 gauss and A(CH3) = 0.095 +/- 0.007 gauss for Fe(C5H4CH3)(2)(+)] are interpreted in terms of a predominant direct delocalization mechanism involving mainly in-plane ring orbital. This interpretation is supported by extended Huckel calculations. Parameters resulting from the present investigation show that for Fe(C5H5)(2)(+) the (2)A(1g) level is below the E-2(2g) by about 200 cm(-1), which contradicts recent optical spectra analyses. The possibility that the former is not a pure electronic level but a vibronic one of symmetry A(1g), resulting from Jahn-Teller induced vibrational coupling with E-2(2g), is suggested and discussed.