A MOSSBAUER-EFFECT AND FENSKE-HALL MOLECULAR-ORBITAL STUDY OF THE BONDING IN A SERIES OF ORGANOIRON-COPPER CLUSTERS

The electronic properties of a series of organoiron-copper clusters, N a-2[Cu6Fe4(CO))(16)] (I), Na-3[Cu5Fe4(CO)(16)] (II), Na-3[Cu3Fe3(CO)(1 2)] (III), (P(CH3)(3))(4)](2)[Fe-3(CO)(12)Cu-4(P(CH3)(3))(2)] (IV), [( (C6H11)(3)PCu)(2)Fe(CO)(4)] (V), [((Ph(3)P)(2)Cu)(2)Fe(CO)(4)] (VI), [ (Ph(3)P)(2)CuFe(CO)(3)(NO)] (VII), and [((NH3)(2)Cu)(2)Fe(CO)(4)] (VII I), have been investigated experimentally by the Mossbauer effect and theoretically by Fenske-Hall molecular orbital calculations. The Mossb auer effect hyperfine parameters are sensitive to the variety of bondi ng situations found in these clusters. The Mossbauer effect isomer shi fts observed for these clusters range from -0.141 to -0.037 mm/s at 78 K. The expected increase in the isomer shift with a decrease in the i ron 4s Mulliken atomic orbital population, and a decrease in the Cleme nti effective nuclear charge experienced by the iron 4s electrons is o bserved. The electric field gradients at the iron sites have been calc ulated and compared with the experimental quadrupole splittings which range from 0.191 to 2.497 mm/s at 78K. The valence contribution was fo und to be the major component of the electric field gradient and is di rectly related to the symmetry of the iron electronic environment. The calculated Values of the electric field gradients are also used to co nfirm the Mossbauer effect spectral assignments in II. The calculated metal-metal bond energy decreases as the cluster metal-metal bond leng th increases. The major bonding in these clusters is the iron-copper b ond in which the iron 3d atomic orbitals overlap the copper 4s and 4p atomic orbitals.