Mössbauer and molecular orbital (MO) study of localized and delocalized mixed-valence organo-FeIIFeI bisandwiches

A Mossbauer study of two mixed-valence FeIIFeI compounds C5H5Fe(C5H4)2 FeC6(CH3)6 (1) and ¦C6(CH3)6 Fe(C5H4)2 Fe C6(CH3)6¦+ (2+) was carried out from 4.2K to room temperature. Zero-field spectra show two types of iron atoms for1 and one type for2+. Hence1 is a localized mixed-valence complex and2+ a delocalized mixed-valence complexe. High magnetic field spectra for 2+ give a negative sign for the EFG and show the valence electron is delocalized on the two centers. IEHT-MO calculations confirm the results and allow to explain the temperature independence of the quadrupole splitting (QS) of2+.