MOSSBAUER-EFFECT AND FENSKE-HALL MOLECULAR-ORBITAL STUDY OF THE ELECTRONIC-STRUCTURE OF SEVERAL TRINUCLEAR IRON CARBONYL CLUSTERS

Fenske-Hall molecular orbital calculations have been carried out on the trinuclear carbonyl clusters Fe3(CO)12, [Fe3(CO)10CH]-, [Fe3(CO)9CCO]2-, and [Fe2Co(CO)9CCO]-, and their Mossbauer spectra have been measured at 78 K. The iron 4s Mulliken atomic population shows the expected increase as the Mossbauer-effect isomer shift decreases. Further, we find an excellent correlation between the observed isomer shift and the sum of the iron 4s population and the effective nuclear charge they experience. A calculation of the electric field gradients at the iron nuclei, based on the Mulliken atomic charges and the iron wave functions derived from the Fenske-Hall molecular orbital calculations, reveals that the valence contribution by far predominates over the lattice contribution to the electric field gradient at the iron-57 nucleus. There is a rather good correlation between the calculated and observed quadrupole splittings in the iron clusters.