Octahedral ethylenediamine complexes, ring

Fig. 21-3. Diagrams showing the different sets of repulsive interactions that exist between the three different pairs of ring conformations in octahedral ethylenediamine complexes. Broken lines represent the significant repulsive interactions. [Reproduced by permission...

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Octahedral Chelate Complexes.— 2T-Ray analysis of (-l-) g,-c -dicyanobis-(ethylenediamine)cobalt(iii) chloride monohydrate establishes that the absolute configuration of the [Co(CN)2(en)J+ complex cation is a (AA), in conformity with the previous spectroscopic assignment. Both chelate rings are of the ob conformation. The mean Co-N and Co-C distances are 1.97 and 1.87 A, respectively and the Co-C-N units do not deviate significantly from linearity. In a preliminary note the absolute configurations of both complex ions in (—)6s -[Co(N02)2(en)J(+)68 -[Co(mal)jen], where mal = malonato, CHa(C08)2 , are said to be A and all en chelate rings to exhibit the ob conformation. ... 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

Stereoselectivity ond In addition to the dissymmetry generated by the tris(chclaie) structure of octahedral he Conformation of complexes, it is possible to have dissymmetry in the ligand as well. For example. Ihe Chelate Rings gauche confiwmalion of ethylenediamine is dissymetric (Fig. 12.27) and could be... 

Fig. 23.1 Structure of ICo(en)3l3+. It is a six-co-ordinate octahedral complex of ethylenediamine (en) with cobalt (III). The complex has three five-membered rings.

The preparation of transition metal complexes with penten marks the last step in mononuclear octahedral complexes begun by Jorgensen and Werner last century with ethylenediamine. Stuart models indicate that the fifth and sixth amine nitrogens are progressively harder to fit into place on the octahedron. If we assume a five-membered ring strain with each tetrahedral nitrogen of 1.5°, the fourth and fifth rings would have a cumulative strain of 6 and 7.5°, respectively. The ion [Copenten] should exist in four pairs of optically-active isomers ... 

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