MOSSBAUER-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 , HFe4(CO)12(eta2-CH) (I), (PPN)[HFe4(CO)12C] (II), (BzNMe3)2[Fe4(CO)1 2C] (III), (PPN) [RhFe3(CO)12C] (IV), Fe4(CO)13C (V), (PPN)[MnFe3(CO)1 3C] (VI), (PPN)2[CrFe3(CO)13C] (VII), (PPN)2[WFe3(CO)13C] (VIII), (PPN )[CrFe3(CO)13(CH)] (IX), (PPN)[WFe3(CO)13(CH)] (X), HCrFe3(CO)13(CH) ( XI), and HWFe3(CO)13(CH) (XII), have been studied both experimentally by the Mossbauer effect at 78 K and theoretically with Fenske-Hall mol ecular orbital calculations. In these clusters the Mossbauer-effect is omer shifts range from -0.225 to -0.029 mm/s for the unprotonated wing tip 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. T he quadrupole splittings range from 1.026 to 1.829 mm/s for the unprot onated wingtip iron sites, from 0.469 to 1.659 mm/s for the backbone i ron sites, and from 0.831 to 1.183 mm/s for the protonated wingtip iro n sites. The larger observed quadrupole splittings of the wingtip iron atoms indicate that their electronic environment is more distorted th an that of the backbone iron atoms. In the anionic clusters the anioni c charge is found to be delocalized predominately onto the oxygen of t he carbonyl ligands. The carbide and carbonyl ligands become more nega tive 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 4s03d84p0, become s on average 4s0.373d6.744p0.86, with a range from 4s0.343d6.764p0.72 in I to 4s0.383d6.714p0.93 in VI. The isomer shifts of the backbone an d protonated wingtip iron sites are more positive than those of the un protonated wingtip iron site because, for the former, the iron 4s to l igand overlap population is less than for the latter. The iron 4s-elec tron density, as measured experimentally at the iron-57 nucleus by the Mossbauer-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 al so found to decrease as the iron 49 to near-neighbor overlap populatio n increases. The calculated quadrupole splittings, obtained from the F enske-Hall molecular orbital wave functions, agree rather well with th e observed quadrupole splittings.