A Mössbauer Effect Study of the Electronic Structure of Several Tetranuclear Organoiron Clusters

The Mössbauer spectra obtained at 78 K for a series of tetranuclear organoiron clusters reveal well-resolved quadrupole doublets with a range of average isomer shifts which may be related to the charge on the cluster. Single-crystal X-ray structural results are available for each of the clusters studied, and the Mössbauer spectra reveal the expected number of crystallographically distinct iron sites. The spectrum obtained for (PPN)2[Fe4(CO)13] (I), which contains a tetrahedral iron cluster with a triply-bridged carbonyl on one face, reveals a high electronic symmetry for the unique iron site. The remaining clusters have the butterfly structure and Mössbauer spectra which have clearly resolved doublets for the wingtip and backbone iron sites. The isomer shift values observed for each of the iron sites in several of the butterfly clusters is linearly related to the Slater effective nuclear charge experienced by the iron 4s electrons. The effective nuclear charge has been calculated from the orbital occupation values obtained through Fenske-Hall molecular orbital calculations published by Harris and Bradley. The results indicate that the Mössbauer effect isomer shift, when used in conjunction with detailed molecular orbital calculations, provides an accurate map of the electronic charge density distribution in a cluster. Further it provides insight into how this charge distribution changes with changes in the peripheral ligands in such a cluster. © 1986 American Chemical Society