The He I and X-ray photoelectron spectra of the iron tetracarbonyl complexes of methyl, tert-butyl,
trimethylsilyl, and phenyl isocyanide have been obtained. The bonding properties of these isocyanides
are compared with those of carbon monoxide by using the core-binding energy data to interpret the valence
ionization energies. The data show that all the isocyanides have comparable a-donor ability and that, in
the Fe(C0I4CNR complexes, they are significantly weaker u donors than carbon monoxide. The stronger
u-donor character of the coordinated CO ligands is presumably due to the u-a synergism of the Fe-CO
bonding. The net interaction of an isocyanide ligand with the iron d orbitals leaves the energy of the d
orbitals destabilized relative to Fe(C0)5. The data indicate that the relative energies of interaction of the
d orbitals with the filled ligand a orbitals and the empty ligand a* orbitals are different in Fe-CO and
Fe-CNR bonds, with the interactions of the filled ligand a orbitals being relatively more important in
Fe-CNR bonds than in Fe-CO bonds. The net electron withdrawal by a coordinated isocyanide ligand,
although significant, is much lower than that by a coordinated carbonyl ligand. It is clear that the interactions
of transition metals with isocyanides (and probably also with other ligands with similar electronic structures)
cannot be adequately described without considering the combined a-donor, a-acceptor, and a-donor
characters of the ligands
