On the origin of circular dichroism in trigonal dihedral d6 complexes of bidentate ligands containing only sigma-orbitals. A qualitative model based on a density functional theory study of Lambda-[Co(en)3]3+

Time-dependent density functional theory (TD-DFT) in conjunction with qualitative molecular orbital theory is employed to interpret the circular dichroism (CD) spectrum of Lambda-[Co(en)(3)](3+). The simulated spectrum based on TD-DFT is compared in details with the experimental data. Emphasis is put on a qualitative understanding of the origin of optical activity in sigma-bonded trigonal dihedral complexes with bidentate ligands in general. Rotatory strengths associated with the CD bands of both d-d transitions and ligand-to-metal charge transfer (LMCT) are interpreted based on the metal-ligand sigma-interactions. These interactions are in turn analyzed in terms of symmetry unique overlaps between metal d- and symmetry ligand-orbitals as well as the dependence of these overlaps on geometrical parameters of the complex.