On the Origin of the Optical Activity in the d-d Transition Region of Tris-Bidentate Co(III) and Rh(III) Complexes

Time-dependent density functional theory (TD-DFT) has been employed to calculate the rotatory strengths in the d-d transition region for various tris-bidentate Co(III) and Rh(III) complexes. Optimized structural parameters are also reported. Our results confirm a previously proposed relationship between the azimuthal distortion of a complex containing saturated tris(diamine) and its optical activity. Formally d-d transitions are forbidden and should not exhibit optical activity. However, it is shown here that the intensity of these bands originates from a coupling of even ligand combination (participating in the eg type LUMO) and an odd ligand combination (participating in the t2g HOMO). For complexes containing planar unsaturated ligands, the signs of the d-d bands observed from the single-crystal circular and linear dichroisms are in accordance with the TD-DFT predictions. It is shown that by using hypothetical Co (NH3)63+ complexes it is possible to estimate the contribution from the azimuthal distortion to the total rotatory strengths of the saturated tris(diamine) complexes. A discussion is also provided of previous theoretical studies and the way in which these investigations rationalized the optical activity.