Stereospecific effects of asymmetric ligands in cobalt (III) complexes

Abstract

In the study of the effects of asymmetric groups in metal complexes, by far the most commonly used ligand has been propylenediamine. This base is a bidentate ligand and its metal complexes show a close chemical resemblance to those of ethylenediamine, so that any complex of ethylenediamine can be paralleled using propylenediamine. X-ray and infra-red studies show that the metal ligand ring in both ethylenediamine and propylenediamine complexes assumes a non-planar configuration. The atoms are distributed in a staggered conformation so that the angles between bonds deviate as little as possible from the tetrahedral and octahedral angles. This puckering, shown in Fig. 1.11 for the ethylenediamine case, gives rise to isomerism such as that found in the cyclopentane system. The hydrogen atoms attached to the methylene carbons can be either "axial" or "equatorial." Using conformational anlyses Bailar and Corey were able to predict which isomers should be energetically preferred to others in a system such as tris(ethylenediamine)cobalt-(III). They also applied this treatment to the corresponding propylenediamine complex and predicted the predominance of some isomers. (First two paragraphs of introduction).