Templates for Specific Syntheses

The metals that have been most widely used as templates are shown in Table I. Two correlations arise from the syntheses described in Section III. 

An effective template metal ion binds strongly to the donor atoms of the macrocycle or its precursors, e.g., K+ is the most common template for synthesis of crown ethers and forms definite complexes with a wide variety of crown ethers. 

The preferential coordination geometry of the template metal ion determines the nature of the macrocycle that is formed, e.g., Ni2 + and Cu2+ are particularly effective template ions for the synthesis of N-donor macrocycles containing 4 nitrogen atoms that can be arranged in a plane and, hence, form a square planar macrocycle-metal complex. 

The preference of a metal for a particular ligand is not always the overriding consideration as was demonstrated (115) for XII where 

Section of the Periodic Table Showing Metals Known to Act as Templates 

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