Question

What is meant by the chelate effect? Give example also.

Hint:

Extra stability from ring-forming polydentate ligands; compare en versus ammonia complexes.

Answer 1

Step 1 (Definition): When a bidentate or polydentate ligand binds to a central metal ion through two or more donor atoms, it forms a ring structure called a chelate.
Step 2 (Chelate effect): The chelate effect is the extra stability shown by a complex containing chelate (ring-forming) ligands compared with a similar complex containing an equal number of monodentate ligands.
Step 3 (Reason): Ring formation greatly increases the stability, mainly because breaking a chelate needs several bonds to snap at once, and the process is favoured by a positive entropy change (more free particles are released into solution).
Step 4 (Example): \( [Ni(en)_3]^{2+} \) (en = ethylenediamine, a bidentate ligand) is far more stable than \( [Ni(NH_3)_6]^{2+} \), even though both have six \( Ni-N \) bonds. This greater stability is due to the chelate effect.