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 e_g type LUMO) and an odd ligand combination (participating in the t_2g 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.