JEE Main · 2023mediumCORD-011

Which of the following is the correct order of ligand field strength?

Coordination Compounds · Class 12 · JEE Main Previous Year Question

Question

Which of the following is the correct order of ligand field strength?

Options
  1. a

    CO<en<NH3<C2O42−<S2−\mathrm{CO} < en < \mathrm{NH_3} < \mathrm{C_2O_4^{2-}} < \mathrm{S}^{2-}

  2. b

    S2−<C2O42−<NH3<en<CO\mathrm{S}^{2-} < \mathrm{C_2O_4^{2-}} < \mathrm{NH_3} < en < \mathrm{CO}

    ✓
  3. c

    NH3<en<CO<S2−<C2O42−\mathrm{NH_3} < en < \mathrm{CO} < \mathrm{S}^{2-} < \mathrm{C_2O_4^{2-}}

  4. d

    S2−<NH3<en<CO<C2O42−\mathrm{S}^{2-} < \mathrm{NH_3} < en < \mathrm{CO} < \mathrm{C_2O_4^{2-}}

Correct Answerb

S2−<C2O42−<NH3<en<CO\mathrm{S}^{2-} < \mathrm{C_2O_4^{2-}} < \mathrm{NH_3} < en < \mathrm{CO}

Detailed Solution

🧠 Reading the Series Forwards This Time

"Increasing field strength" means walk up the spectrochemical ladder. Anchors:

  • Bottom: S2−\mathrm{S^{2-}} (sulphide donor — diffuse, weak field)
  • Then oxalate (C2O42−\mathrm{C_2O_4^{2-}}, O donor)
  • Then NH3\mathrm{NH_3} (N donor)
  • Then en (chelating N donor — stronger than NH3\mathrm{NH_3} due to chelate effect)
  • Top: CO\mathrm{CO} (best π\pi-acceptor)

That sequence is exactly option (2).

🗺️ Eliminate the Distractors

(1) Starts with CO\mathrm{CO} as weakest. Wrong direction. (2) S2−<C2O42−<NH3<en<CO\mathrm{S^{2-} < C_2O_4^{2-} < NH_3 < en < CO}. Walks the ladder upward. ✓ (3) Places C2O42−\mathrm{C_2O_4^{2-}} above CO\mathrm{CO}. Oxalate is a weaker field ligand than CO\mathrm{CO} — wrong. (4) Places S2−\mathrm{S^{2-}} below NH3\mathrm{NH_3} correctly but ends with C2O42−\mathrm{C_2O_4^{2-}} as strongest. Wrong.

⚡ The "en > NH₃" Anchor

en beats NH3\mathrm{NH_3} slightly because of the chelate effect — two N donors locked together. Memorise that single inequality and most "increasing strength" MCQs collapse to one option.

⚠️ Don't Auto-Apply HSAB

In acid–base intuition, soft-soft pairings like S2−\mathrm{S^{2-}} with late transition metals form very stable complexes. Stability ≠\neq field strength, though. S2−\mathrm{S^{2-}} is still a weak crystal-field ligand because the donation is diffuse and reduces Δ\Delta. Stability constants and Δ\Delta are different physics — keep them separated.

Answer: (2) S2−<C2O42−<NH3<en<CO\boxed{\text{Answer: (2) } \mathrm{S^{2-} < C_2O_4^{2-} < NH_3 < en < CO}}

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