Stereospecific induction

Abstract

The first resolution of an inorganic transition metal complex by Werner (1911), over fifty years ago, made possible the study of the optical activity of the metal chromophore. In his classic studies, Werner (1912 a ) resolved the cis -bis-ethylene-diamine and tris-ethylenediamine systems of tervalent cobalt. Since the absolute configuration of no dissymmetric complex was known, early studies were devoted mainly to correlating the configurations of structurally related complexes with one another. A number of rules were proposed for this purpose, the most notable of which (Werner 1912 b ) asserted that when a common resolving reagent was employed, similar complexes forming the less soluble diasterioisomers had the same absolute configuration. In addition, it was tacitly assumed that structurally similar complexes of the same central metal would give similar rotatory dispersion and circular dichroism spectra. Thus, the bis -ethylenediamine (en) complexes of cobalt (III) of the type cis -[Co en 2 X 2 ] n + were shown (Mathieu 1936) to have rotatory dispersion and circular dichroism spectra, which in most cases were consistent with the diasteriomeric solubility criterion. It was only with the re-emergence of the crystal field theory (Bethe 1929), the determination of the absolute configuration of the tris-ethylenediamine cobalt (III) ion [Co en 3 ] 3+ by X-ray diffraction (Saito, Nakatsu, Shiro & Kuroya 1957), and the use (Corey & Bailar 1959) of optically active bidentate ligands of known absolute configuration that the transition metal chromophore had a reasonable spectroscopic base and in the case of tris-bidentate systems an absolute stereo­-chemical reference point. As yet, no absolute configuration of a bis -bidentate system has been determined by the X-ray method, and there seems no simple chemical method by which the tris- and bis-bidentate complexes can be related; although recently (McCaffery, Mason & Norman 1965) a spectroscopically based relation has been put forward.