Stability of Chelates Chelate and Macrocyclic Effects

Recall that chelates are particular complexes in which a multidentate coordinate 

The striking characteristic of these particular complexes is their very high stability. They are incomparably more stable than complexes built with monodentate ligands of basicity (and probably of nucleophilicity) close to that exhibited by the teeth of the chelate and built with the same central ion. To be convinced, it is sufficient to compare the global stability constants Pg of the hexamminenickel(II) [Ni(NH3)6] + complex and P3 of the triethylenediamminenickel(II) [Ni(en)3] +, which is a chelate (Fig. 24.5). In both compounds, it is the same central ion, and one may easily admit that the basicities and the nucleophilicities of both sorts of ligands are about the same. The difference between the stability constants, P3 = 10 and Pe = 3.1 x 10 , is tremendous. 

The difference in stabilities exhibited by the complex that could, a priori, be considered analogous to the chelate and by the chelate itself is called the chelate effect. Concerning the stability of the chelates, Schwarzenbach underlined the following stability factors  

Concerning the role of the pentagonal cycles in the stability of chelates, we can note that cycles with four summits are less stable than those with five summits. It is the same state of affairs with cycles possessing seven summits compared to cycles with six summits and a fortiori for cycles with more than seven summits  

the nature of the atoms that donate electronic pairs. Oxygen and nitrogen atoms give the most stable chelates  

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