Vapor-phase and solution-phase UV spectroscopic studies of η5-pentadienyl derivatives of iron and ruthenium

Abstract

The electronic absorption spectra of the ‘half-open metallocenes’, (Cp)(C 7H 11)Fe, (Cp)(C 7H 11)Ru, (Cp*)(C 7H 11)Ru (Cp= η 5-cyclopentadienyl, Cp*= η 5-pentamethylcyclopentadienyl, C 7H 11= η 5-2,4-dimethylpentadienyl), and ‘open metallocenes’, (C 7H 11) 2Fe, (C 7H 11) 2Ru, have been measured in the vapor phase and in n-pentane solution and have been compared with the spectrum of the ‘closed ferrocene’ (Cp)(Cp*)Fe. The vapor-phase spectrum of (Cp)(Cp*)Fe reveals two sharp absorption bands arising from the 3d z2→R4s and 3d z2→R4d transitions. These bands are absent in the solution spectrum. Similarly, the spectra of iron and ruthenium η 5-pentadienyl complexes show absorption features which disappear on going from the vapor to the solution phase. These features were interpreted as being due to Rydberg excitations originating at the metal d z2 orbital. The estimated term values indicate that Rydberg bands in the spectra of ‘half-open metallocenes’ and ‘open metallocenes’ most likely correspond to transitions from the molecular orbital responsible for the first ionization peak in the photoelectron spectra to the lowest Rydberg s, p, and d levels. Gaussian analysis shows that Rydberg bands are successively broadened on going from the ‘closed ferrocene’ to ‘half-open metallocenes’ and then to ‘open metallocenes’. This broadening can be interpreted as a result of an increase in mixing of the metal d z2 orbital with ligand functions in this group of compounds.