Mössbauer-effect and Fenske-Hall Molecular Orbital Study of the Electronic Properties of a Series of Organoiron Butterfly Clusters

The electronic properties of a series of organoiron butterfly clusters, HFe 4(CO) 12(η 2-CH) (I), (PPN)[HFe 4(CO) 12C] (II), (BzNMe 3) 2[Fe 4(CO) 12C] (III), (PPN)[RhFe 3(CO) 12C] (IV), Fe 4(CO) 13C (V), (PPN)[MnFe 3(CO) 13C] (VI), (PPN) 2[CrFe 3(CO) 13C] (VII), (PPN) 2[WFe 3(CO) 13C] (VIII), (PPN)[CrFe 3(CO) 13(CH)] (IX), (PPN)[WFe 3(CO) 13(CH)] (X), HCrFe 3(CO) 13(CH) (XI), and HWFe 3(CO) 13(CH) (XII), have been studied both experimentally by the Mössbauer effect at 78 K and theoretically with Fenske-Hall molecular orbital calculations. In these clusters the Mössbauer-effect isomer shifts range from -0.225 to -0.029 mm/s for the unprotonated wingtip iron sites, from -0.018 to 0.141 mm/s for the backbone iron sites, and from 0.009 to 0.124 mm/s for the protonated wingtip iron sites. The quadrupole splittings range from 1.026 to 1.829 mm/s for the unprotonated wingtip iron sites, from 0.469 to 1.659 mm/s for the backbone iron sites, and from 0.831 to 1.183 mm/s for the protonated wingtip iron sites. The larger observed quadrupole splittings of the wingtip iron atoms indicate that their electronic environment is more distorted than that of the backbone iron atoms. In the anionic clusters the anionic charge is found to be delocalized predominately onto the oxygen of the carbonyl ligands. The carbide and carbonyl ligands become more negative as the electronegativity of the cluster heterometal decreases. As the carbide and carbonyl ligands donate electron density to the metal, the iron electronic configuration, which begins at 4s 03d 84p 0, becomes on average 4s 0.373d 6.744p 0.85, with a range from 4s 0.343d 6.764p 0.72 in I to 4s 0.383d 6.714p 0.93 in VI. The isomer shifts of the backbone and protonated wingtip iron sites are more positive than those of the unprotonated wingtip iron site because, for the former, the iron 4s to ligand overlap population is less than for the latter. The iron 4s-electron density, as measured experimentally at the iron-57 nucleus by the Mössbauer-effect isomer shift, decreases as expected with an increase in the sum of the iron 4s Mulliken atomic orbital population and the Clementi and Raimondi effective nuclear charge. The isomer shift is also found to decrease as the iron 4s to near-neighbor overlap population increases. The calculated quadrupole splittings, obtained from the Fenske-Hall molecular orbital wave functions, agree rather well with the observed quadrupole splittings. © 1993 American Chemical Society