Chiral octahedra

Ethylenediamine always spans adjacent corners of an octahedron, but the Cl- ligands can be on either adjacent or opposite corners. Therefore, there are two diastereoisomers, cis and trans. Because the trans isomer has several symmetry planes—one cuts through the Co and the en ligands—it is achiral and has no enantiomers. The cis isomer, however, is chiral and exists as a pair of enantiomers that are nonidentical mirror images. 

Fig. 32 Assembly of the molecular octahedron 60 from six end-capped palladium(II) corners and four C2V-symmetric ligands 59. The overall C2-chirality of 60 arises from the non-symmetrical arrangement of 59. Reprinted with permission from [56], Copyright 2002 The Chemical Society of Japan...

Closely related to the football ligands are the so-called sepulchrate ligands. One can be formed by the condensation of formaldehyde and ammonia onto the nitrogen atoms of tris(ethylenediamine)cobalt(llI). This results in tris(methylene)amino caps on opposite faces of the coordination octahedron. If the synthesis utilizes one of the (A, A)-enantiomers, the chirality of the complex is retained. Furthermore, the complex may be reduced to the corresponding cobalt(II) cation and reoxidized to co-balt(III) without loss of chirality. This is particularly unusual in that, as we shall see in the following chapter, cobalt(ll) complexes are quite labile in contrast to the stability of cobalt(III) complexes. Once again the extra stability of polydentate complexes is demonstrated. 

The complexity order of Archimedean solids in terms of the solid angle of their vertices is280 TT < CO < TC < TO < RCO < ID < TCO < TD < TCO < RID < TID. The two chiral Archimedean solids (snub octahedron, snub icosidodecahedron) were not considered. This order disagree with all four complexity given above, except in the case of the truncated tetrahedron which is predicted to be the least complex of all Archimedean solids. This discrepancy is perhaps due to different bases of the compared complexity orders the above orders being the result of 2D representation and the Balaban-Bonchev order of 3D structure of Archimedean solids. 

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