Hexadentate Crown Thioethers

Black and McLean pointed out that a hexadentate ligand can yield either of two isomeric octahedral complexes of meso or racemic stereochemistry (Fig. 9) [52, 53]. These differ primarily in that the former consists entirely of facially coordinating tridentate loops the latter comprises two meridional loops (i.e., those in which three adjacent donor atoms, as well as the metal ion, lie in a plane). In all encapsulated complexes reported to date, 18S6 coordinates in the centrosymmetric meso form. This behavior contrasts with the racemic coordination found for corresponding hexaaza macro-cycles [144-147]. 

This difference apparently arises at least in part from the differing propensity of the two ligands to adopt gauche placement at the C-heteroatom bonds [80, 86]. In 

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