MOSSBAUER STUDY AND MOLECULAR-ORBITAL CALCULATIONS ON SOME BIMETALLICDERIVATIVES OF FERROCENE AND FERRICINIUM

We have studied by absorption Mossbauer spectroscopy, some bridged der ivatives of biferrocene, unoxidized (Fe(II)-Fe(II)) and mono-oxidized (Fe(II)-Fe(III)). The mixed-valence species exhibits a quasi-delocaliz ed behavior at the Mossbauer timescale (approximately 10(-8) s), with a splitting of the lines which is interpreted in terms of a partial va lence trapping. This in turn reveals intramolecular properties at vari ance from those of the unbridged cation. SCC-Xalpha MO calculations ha ve been performed; they correctly reproduce the measured quadruple spl ittings and provide evidence that the HOMO of the mixed-valence comple x is of d(xy), d(x2-y2) type. Some consequences on the mixed-valence p roperties are discussed.