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Q5.26:
What is meant by the chelate effect? Give an example.

Solution :

1. Definition of the Chelate Effect

The chelate effect refers to the significantly enhanced thermodynamic stability of a coordination complex containing one or more chelate rings (formed by polydentate ligands) compared to a complex containing an equivalent number of monodentate ligands. A complex is considered "chelated" when a polydentate ligand binds to a single metal ion at two or more donor sites, creating a closed-loop structure [Per IUPAC definition of coordination stability].

2. Thermodynamic Basis

The stability is driven by the change in Gibbs free energy ($\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$). The formation of chelated complexes is largely entropy-driven ($\Delta S^\circ > 0$). When polydentate ligands displace monodentate ligands, the number of free particles in the solution increases, resulting in a positive entropy change [Per the Second Law of Thermodynamics].

3. Comparative Analysis

Consider the exchange of ammonia ligands with ethylenediamine ($en$) in a copper(II) complex:

Reaction Species Count (Reactants vs Products)
$[Cu(NH_3)_4]^{2+} + 2en \rightarrow [Cu(en)_2]^{2+} + 4NH_3$ 3 particles vs 5 particles

The net increase in the number of free molecules ($\Delta n = +2$) leads to a large increase in translational entropy, making the formation of the chelated product thermodynamically favorable.

4. Visual Representation of Chelate Ring

Cu NH2 NH2 CH2-CH2 Structure: [Cu(en)2]2+

5. Practical Example

The formation of the bis(ethylenediamine)copper(II) ion from the tetraamminecopper(II) ion is the classic textbook illustration of the chelate effect. Because ethylenediamine ($H_2N-CH_2-CH_2-NH_2$) occupies two coordination sites, the ring structure restricts the degrees of freedom of the ligand, and the displacement of multiple monodentate molecules by a single polydentate molecule increases the overall disorder of the system [Per the Statistical-Mechanical interpretation of Entropy].

Final Solution: The chelate effect is the observation that coordination complexes with polydentate ligands are more stable than those with monodentate ligands due to a favorable positive entropy change ($\Delta S > 0$) occurring during ligand exchange. An example is the formation of $[Cu(en)_2]^{2+}$ from $[Cu(NH_3)_4]^{2+}$.


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