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.