Optical rotatory power of co-ordination compounds. Part XII. Spectroscopic and configurational assignments for the tris-bipyridyl and -phenanthroline complexes of the di- and tri-valent iron-group metal ions

Absorption and circular dichroism spectra are reported for the tris-bipyridyl and the -phenanthroline (phen) complexes of iron(II), ruthenium-(II) and -(III), and osmium-(II) and -(III). The results are analysed in terms of the exciton model for optical activity and the perturbing effects of π-electron exchange between the ligands and between the ligands and the metal ion. It is found that the principal spectroscopic and stereochemical conclusions of the exciton theory are qualitatively unaffected by the perturbations, provided that the complete monopole exciton energy is considered rather than that evaluated in the point-dipole approximation. Metal–ligand π-bonding is shown to be more important in the di- than the tri-valent complexes, the apparent exciton-splitting being diminshed in both cases. Ligand–ligand bonding may enhance or decrease the exciton splitting, depending upon the symmetry of the ligand π-orbitals with respect to rotation about the two-fold (short) ligand axis. The p-band in the spectra of bipyridyl and phenanthroline is found to be predominantly long-axis polarised, although there is a short-axis component in the latter case due to the overlapping α-band. The higher energy β-band is shown to be short-axis polarised in both cases. The main circular dichroism of the complexes in the visible region is found to reflect the exciton dichroism of the u.v. region, owing to the combination of long-axis polarised charge-transfer transitions with ligand excitations of the same polarisation. The optical isomers studied are assigned the M(C3) stereochemical configuration established for (–)-[Fe phen3]2+ by X-ray analysis.