Do I Look Big on That - Chelate Ring Size

What we find is that there is a preferred chelate ring size as the ring size rises, there is a rise in stability of the assembled complex, and then a fall as the ring continues to grow. This trend depends on a number of factors, such as what metal ion, what donor groups, and what ligand framework is involved. Nevertheless, for the common lighter metals (first row of the periodic d block) the trend is fairly consistent  

Examples of from three- to seven-membered chelate rings. Note how ring size impacts on the L—M—L intraligand angle, shown in the lower set. 

In all cases above, the trend 5-membered 6-membered 7-membered ring is preserved. The size of the metal ion and its preferred bond lengths also play a role - for example while the trend is consistent for the lighter metals, larger metal ions may prefer a different ring size. There is another trend exhibited here that we will return to later also - a consistent variation with metal, irrespective of ligand. 

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