Ligand-polarization models for the optical activity of chiral cobalt(III) complexes octahedrally-coordinated with monodentate ligands

Abstract

The optical activity exhibited by chiral monodentate complexes of the type all-cis-[Co(A)₂(B)₂(C)₂] over the visible wavelength region is investigated theoretically, employing a third-order and a fourth-order ligand-polarization model. The third-order model considers a secondary and a tertiary coulombic correlation between the induced ligand dipoles, following the primary orientation of those dipoles by the field of the hexadecapole moment of the metal ion transition. The third-order rotational strengths vanish if the coordination octahedron of the complex is geometrically regular, but not those of the fourth-order model, which is based upon the pairwise mixing of the three components of the octahedral ¹ A ₁ →¹ T ₁ cobalt(III) d-electron transition, mediated by the coulombic potential between the individually correlated induced dipoles in different ligands. The sum of the third- and fourth-order contributions reproduce the signs, and a significant fraction of the magnitude, of the observed rotational strengths exhibited by a representative chiral complex.