Sepulchrates chelating ligands

A second consequence of this sequential mechanism is even more surprising. The geometry about the metal centre in a sepulchrate is close to octahedral, and this is emphasised in Fig. 7-17. We discussed some of the stereochemical properties of complexes with three didentate chelating ligands in Chapter 2, and in the same way that the [Co(en)3]3+ cation may exist in the two enantiomeric A and A forms, so may the sepulchrate complex. These two enantiomers are shown in Fig. 7-17. The reaction of [Co(en)3]3+ with formaldehyde... 

Not only does chelation make the complex more stable, but it also forces the donor atoms to take up adjacent or cis sites in the resulting complex. Equation (2) shows how displacement of a chelating carbonate ion gives the unusual cis dichloride product instead of the thermodynamically more stable trans dichloride. Polydentate chelating ligands with three or more donor atoms also exist. Macrocyclic ligands, such as (4) and (5), confer an additional increment in the formation constant (the macrocyclic effect) they have been given trivial names, such as cryptates (4) and sepulchrates (5).i... 

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