The Chelate Effect refers to the increased stability of complexes containing polydentate (chelating) ligands compared to monodentate ones.

• When a chelating ligand (like ethylenediamine) replaces two monodentate ligands (like $N{H}_3$): $[M(N{H}_3{)}_2]+en\rightleftharpoons [M(en)]+2N{H}_3$
• The number of particles in the solution increases (from 2 reactants to 3 products).
• This leads to an increase in entropy ($\Delta S>0$), making the Gibbs free energy $\Delta G=\Delta H-T\Delta S$ more negative.
• Therefore, the chelate effect is primarily entropy-driven.