The spectra of complexes of conjugated ligands. Part III. Charge-transfer spectra of bisnioximebis(substituted pyridine)iron(II) complexes

Abstract

A series of bisnioximebis(substituted pyridine)iron(II) complexes has been prepared and analysed, and their visible and near-u.v. spectra have been measured in solution and in the solid state. The compounds exhibit low-energy charge-transfer bands from iron(II) to nioxime and to pyridine. The effects of methyl substituents on the energies of these bands are discussed by using the approach developed1 for phenanthroline complexes, in which the shifts are explained as due to a combination of changes in the ligand molecular-orbital and d-orbital energies. As in the case of phenanthroline, shifts on successive substitution are additive to within experimental error; their numerical values are quite consistent with those in the phenanthroline complexes. The pyridine-acceptor molecular orbital is identified. The areas of both the charge-transfer bands are used to obtain values for the iron-nitrogen resonance integral; the two values agree remarkably well. The charge-transfer band to pyridine shows a large solvent shift, explained by the changing orientation of the pyridine rings.