Chelation ligand shape

These NOE studies teach us that many successful P (or N...etc) auxiliaries possess a relatively rigid and intrusive chiral pocket [105, 107]. The shape of this pocket is a function of the individual chelate ligand, i. e., there is no one successful shape. 

Although most of the chelate ligands you will encounter have a carbon backbone and heteroatoms like nitrogen, oxygen or sulfur as donors, this is not essential. This can best be illustrated by comparing the two pairs of chelated systems in Figure 2.19, which have identical ring sizes and shapes, yet one of each pair contains no carbon atoms at all. 

How a pre-defined appropriate ligand shape (pre-organization) provides chelation at less energy cost than in circumstances where energy-demanding ligand re-arrangement must accompany chelation. 

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