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ChemistryMediumClass 12

Stability ā€” Chelate effect, stability constants

Coordination Compounds

15

JEE Qs

8%

Hard

60

min

Focus on understanding the thermodynamic basis (entropy) of the chelate effect and how to compare stability using given stability constant values and ligand properties.

šŸ§® Key Formulas

K_n = [ML_n] / ([ML_(n-1)][L]) (Stepwise stability constant)
Ī²_n = [ML_n] / ([M][L]^n) (Overall stability constant)
Ī²_n = K_1 * K_2 * ... * K_n
Ī”GĀ° = -RT ln K_eq (where K_eq can be K_n or Ī²_n)

āœ… Key Points for JEE

  • 1The Chelate Effect is the enhanced stability of a coordination complex containing a chelate ligand (polydentate ligand) compared to an analogous complex with only monodentate ligands.
  • 2The primary thermodynamic reason for the chelate effect is a favorable entropy change (Ī”S > 0), as one polydentate ligand replaces multiple solvent molecules, leading to an increase in the number of particles in solution.
  • 3Stability constants (stepwise K_n and overall Ī²_n) are equilibrium constants for the formation of a complex; a higher value indicates greater thermodynamic stability.
  • 4Chelate rings of 5 or 6 members are generally the most stable due to minimal ring strain, while 4-membered rings are highly strained and larger rings offer less entropic advantage.
  • 5Factors increasing complex stability include: higher charge on the central metal ion, smaller size of the metal ion (higher charge density), stronger basicity/donor strength of the ligand, and the presence of chelate rings.

āš ļø Common Mistakes

  • āœ•Confusing thermodynamic stability (equilibrium constants) with kinetic stability (lability/inertness) of coordination complexes.
  • āœ•Incorrectly assuming all chelate complexes are equally stable; the number and size of chelate rings significantly impact stability.
  • āœ•Overlooking the entropic contribution to the chelate effect, often focusing solely on the enthalpy of bond formation.
  • āœ•Mistaking stepwise stability constants (K_n) for overall stability constants (Ī²_n) or miscalculating Ī²_n from K_n values.

šŸ“ Practice Questions

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Q74.In the Claisen-Schmidt reaction to prepare, dibenzalacetone from 5.3 g of benzaldehyde, a total of 3.51 g of product was obtained. The percentage yield in this reaction was ______ %.

2025Ā·NumericalHard

Q61.The calculated spin-only magnetic moments of K3 [Fe(OH)6] and K4 [Fe(OH)6] respectively are : (1) 3.87 and 4.90 B.M. (2) 4.90 and 5.92 B.M. (3) 4.90 and 4.90 B.M. (4) 5.92 and 4.90 B.M.

2025Ā·MCQMedium

Q54.Identify the homoleptic complexes with odd number of d electrons in the central metal : (A) [FeO4]2āˆ’ (B) [Fe(CN)6]3āˆ’ (C) [Fe(CN)5NO]2āˆ’ (D) [CoCl4]2āˆ’ (E) [Co(H2O)3 F3] Choose the correct answer from the options given below : (1) (A), (B) and (D) only (2) (C) and (E) only (3) (B) and (D) only (4) (A), (C) and (E) only

2025Ā·MCQMedium

Q68.In which of the following complexes the CFSE, Ī”o will be equal to zero? (1) [Fe(en)3]Cl3 (2) K4 [Fe(CN)6] (3) [Fe(NH3)6]Br2 (4) K3 [Fe(SCN)6]

2025Ā·MCQMedium

Q70.From the magnetic behaviour of [NiCl4]2āˆ’ (paramagnetic) and [Ni(CO)4] (diamagnetic), choose the correct geometry and oxidation state. (1) [NiCl4]2āˆ’: NiII , tetrahedral [Ni(CO)4] : NiII , (2) [NiCl4]2āˆ’ : NiII , square planar [Ni(CO)4] : Ni(0) square planar , square planar (3) [NiCl4]2āˆ’: NiII , tetrahedral [Ni(CO)4] : Ni(0), (4) [NiCl4]2āˆ’: Ni(0), tetrahedral [Ni(CO)4] : Ni(0), tetrahedral square planar

2025Ā·MCQHard

Q53.The correct order of the following complexes in terms of their crystal field stabilization energies is : (1) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(en)3]3+ < [Co(NH3)6]3+ (2) [Co(NH3)6]2+ < [Co(NH3)6]3+ < [Co(NH3)4]2+ < [Co(en)3]3+ (3) [Co(en)3]3+ < [Co(NH3)6]3+ < [Co(NH3)6]2+ < [Co(NH3)4]2+ (4) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(NH3)6]3+ < [Co(en)3]3+

2025Ā·MCQHard

NCERT Chapters

  • Class 12 Chemistry Ch 9: Coordination Compounds