The spectra of complexes of conjugated ligands. Part II. Charge-transfer in substituted phenanthroline complexes: intensities

Abstract

Measurement of the charge-transfer band of tris(phenanthroline)iron(II) at low temperature provides supports for the suggestion that its fine structure arises from a vibrational progression in the excited state. A one-electron theory of charge-transfer is applied to the charge-transfer bands of ferrous, ferric, and cuprous phenanthroline complexes. The energies of the d-orbitals are calculated, and possible reasons for the involvement of unexpected ligand molecular orbitals are discussed. The origin of the intensities is then considered, and it is suggested that the main source of intensity is the transition moment resulting from the transfer of charge itself, though borrowing from internal ferrous and cuprous (not ferric) transitions, and borrowing from internal ligand transitions may be appreciable. Metal-ligand resonance and overlap integrals are estimated from the data. The effects of substitution on extinction coefficients are additive for cuprous complexes. The effects of substitution in each position are extracted statistically; the results are not in perfect agreement with the predictions derived from any of the sources of intensity considered, but do not contradict the assignment mentioned above.