Thermodynamic Origins of the Chelate Effect

We may relate the equilibrium constant to the free energy change for a reaction (Eqs. 8.5 - 8.7). 

The various thermodynamic terms for the formation of 1 1 complexes of transition metals with 1,2-diaminoethane are presented in Table 8-2. 

In each case, both the entropy and enthalpy terms favour the formation of the chelated complex, regardless of the t/-electron configuration. Note, however, that outside the d block, i.e. with alkaline earths and other main group metals, it is often found that the entropy term is dominant. 

In some cases, the enthalpy term may actually oppose the formation of the chelated complex, although the entropy term outweighs it to give an overall favourable free energy term. In general, this situation is the exception rather than the rule. 

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