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 Fe 3(CO) 12, [Fe 3(CO) 10CH] -, [Fe 3(CO) 9CCO] 2-, and [Fe 2Co(CO) 9CCO] -, and their Mössbauer spectra have been measured at 78 K. The iron 4s Mulliken atomic population shows the expected increase as the Mössbauer-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. © 1993 American Chemical Society.