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JEE MAIN Chemistry QUESTION #1004
Question 1
The correct order of the following complexes in terms of their crystal field stabilization energies (CFSE) is:
  • \([\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(en)}_3]^{3+} < [\text{Co(NH}_3)_6]^{3+}\)
  • \([\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(NH}_3)_4]^{2+} < [\text{Co(en)}_3]^{3+}\)
  • \([\text{Co(en)}_3]^{3+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_4]^{2+}\)
  • \([\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+}\)✔️
Correct Answer Explanation
CFSE depends on oxidation state and ligand field strength. \(\text{Co}^{2+}\) complexes have lower CFSE than \(\text{Co}^{3+}\). Among \(\text{Co}^{3+}\): en (bidentate) produces a stronger field than \(\text{NH}_3\) due to chelate effect. Tetrahedral \([\text{Co(NH}_3)_4]^{2+}\) has the lowest CFSE \((= 0.4\Delta_t)\). Order: \([\text{Co(NH}_3)_4]^{2+} < [\text{Co(NH}_3)_6]^{2+} < [\text{Co(NH}_3)_6]^{3+} < [\text{Co(en)}_3]^{3+}\).