JEE Main · 2024mediumCORD-002

The correct order of ligands arranged in increasing field strength is:

Coordination Compounds · Class 12 · JEE Main Previous Year Question

Question

The correct order of ligands arranged in increasing field strength is:

Options
  1. a

    F−<Br−<I−<NH3\mathrm{F}^- < \mathrm{Br}^- < \mathrm{I}^- < \mathrm{NH_3}

  2. b

    Br−<F−<H2O<NH3\mathrm{Br}^- < \mathrm{F}^- < \mathrm{H_2O} < \mathrm{NH_3}

    ✓
  3. c

    H2O<OH−<CN−<NH3\mathrm{H_2O} < \mathrm{OH}^- < \mathrm{CN}^- < \mathrm{NH_3}

  4. d

    Cl−<OH−<Br−<CN−\mathrm{Cl}^- < \mathrm{OH}^- < \mathrm{Br}^- < \mathrm{CN}^-

Correct Answerb

Br−<F−<H2O<NH3\mathrm{Br}^- < \mathrm{F}^- < \mathrm{H_2O} < \mathrm{NH_3}

Detailed Solution

🧠 One Trick: Halide Order Is Backwards

This question hides one trap inside an easy one. The easy part — NH3\mathrm{NH_3} beats H2O\mathrm{H_2O} beats halides — almost everyone gets right. The trap is the halide ordering itself: F−>Cl−>Br−>I−\mathrm{F^- > Cl^- > Br^- > I^-}. Smaller halide = stronger field. The standard "increasing size, increasing strength" intuition from acid–base chemistry is flipped here.

🗺️ Two-Stage Filter

Stage 1 — across donor types. Anywhere NH3\mathrm{NH_3} (N-donor) appears next to H2O\mathrm{H_2O} (O-donor), NH3\mathrm{NH_3} should be on the strong side.

Stage 2 — within halides. F−\mathrm{F^-} is stronger than Br−\mathrm{Br^-}, not weaker.

Apply both:

(1) F−<Br−<I−<NH3\mathrm{F^- < Br^- < I^- < NH_3} — halide order is fully reversed. Reject.

(2) Br−<F−<H2O<NH3\mathrm{Br^- < F^- < H_2O < NH_3} — halide order correct, H2O<NH3\mathrm{H_2O < NH_3} correct. Survives both stages.

(3) Places CN−\mathrm{CN^-} below NH3\mathrm{NH_3}. CN−\mathrm{CN^-} is one of the strongest ligands — it cannot sit below NH3\mathrm{NH_3}. Reject.

(4) Places Br−\mathrm{Br^-} above Cl−\mathrm{Cl^-} — same halide-flip mistake.

⚡ The 5-Second Filter

Memorise just one fragment: I−<Br−<Cl−<F−<OH−<H2O<NH3<en<CN−<CO\mathrm{I^- < Br^- < Cl^- < F^- < OH^- < H_2O < NH_3 < en < CN^- < CO}. Most JEE/NEET MCQs are won simply by checking whether the option respects this string.

⚠️ The Halide Reversal

The halide reversal trips students because in acid strength, HI>HBr>HCl>HF\mathrm{HI > HBr > HCl > HF} (bigger halide, stronger acid). In ligand field strength the order is reversed: smaller halide = stronger field. Different physics — σ\sigma-donation efficiency vs proton dissociation. Don't import one trend into the other.

Answer: (2) Br−<F−<H2O<NH3\boxed{\text{Answer: (2) } \mathrm{Br^- < F^- < H_2O < NH_3}}

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