Optical rotatory power of co-ordination compounds. Part XVI. Intermediate exciton coupling in the circular dichroism of trisbipyridyl complexes

Abstract

The circular dichroism of the trisbipyridyl complex of nickel(II) and of other transition-metal ions in the frequency region of the ligand π-electron absorption has been re-examined using an intermediate exciton coupling treatment in order to account for the observed vibronic structure. Contrary to an earlier report, it is found that the exciton splitting energy required to account for the observed magnitude of the circular dichroism and for the vibronic structure has a value comparable to that used in the strong-coupling treatment. For both the intermediate and the strong coupling method, the exciton energy is positive, giving positive and negative exciton c.d. bands at lower and higher frequency, respectively, in the case of complexes with the Λ-configuration. The analogous exciton c.d. absorption of the trisphenanthroline–metal complexes is found to approximate generally to the strong-coupling case.