The spectra of complexes of conjugated ligands. Part I. Charge-transfer in phenanthroline complexes: energy shifts on substitution

Abstract

A theoretical treatment is given of the charge-transfer bands of ferrous, ferric, and cuprous phenanthroline complexes. Methyl substituents are shown to cause additive shifts in the band frequency. The best values of the shifts associated with substitution are extracted statistically and interpreted by Hückel molecular orbital theory. It is necessary to assume that the methyl groups perturb the molecule both by changing the energy of the π-molecular orbital involved in charge-transfer, and by changing the energy of the d-orbitals. The latter effect can be accounted for as a purely inductive (σ) effect, produced by the change in the σ-donor power of the ligands, which can be assessed theoretically or experimentally. This treatment provides an assignment of the molecular orbital involved in charge-transfer. Furthermore, it is shown that the methyl group acts both inductively and hyperconjugatively; appropriate parameters are obtained from the data and compared with results from other sources. Finally, the relations between d-orbital energy and ligand σ-donor power are discussed.