The Optical Activity of Lanthanide and Transition Metal Complexes

Abstract

The optical activity of chiral transition metal complexes has attracted the attention of coordination chemists and molecular spectroscopists for many years. The optical rotatory properties of coordination compounds have played a prominent and venerable role in the development of inorganic stereochemical analysis and structure elucidation, and experimental and theoretical research in this area has been particularly intense over the past 20 years. The first observation of circular dichroism (CD) in the visible absorption bands of transition metal complexes was reported by Cotton in 1895.1 Over the period 1911–1919, Werner2 resolved into optical isomers a wide variety of bis- and tris-chelated complexes containing achiral ligands and transition metal ions from each of the three transition metal series of the periodic table. This work established the octahedral structure of hexa-coordinated complexes and posed the problems of molecular stereochemistry and absolute configuration of metal coordination compounds. The optical rotatory properties of the Werner complexes were the subject of considerable study in the 1930’s, most notably by Jaeger,3 Mathieu,4 and Kuhn.5–7 The first purely theoretical examination of the origins of optical activity in chiral transition metal complexes was made by Kuhn and Bein.5, 6 Their treatment of these systems was based on the general coupled-oscillator model of molecular optical activity proposed earlier by Kuhn.8–10 This model was purely classical and it was used to relate the absolute configuration to the sign and the form of the visible-region Cotton effects observed for tris-chelated complexes.