JEE Main · 2024mediumCORD-001

The correct sequence of ligands in the order of decreasing field strength is:

Coordination Compounds · Class 12 · JEE Main Previous Year Question

Question

The correct sequence of ligands in the order of decreasing field strength is:

Options
  1. a

    NCS−>EDTA4−>CN−>CO\mathrm{NCS}^- > \mathrm{EDTA}^{4-} > \mathrm{CN}^- > \mathrm{CO}

  2. b

    CO>H2O>F−>S2−\mathrm{CO} > \mathrm{H_2O} > \mathrm{F}^- > \mathrm{S}^{2-}

    ✓
  3. c

    S2−>OH−>EDTA4−>CO\mathrm{S}^{2-} > \mathrm{OH}^- > \mathrm{EDTA}^{4-} > \mathrm{CO}

  4. d

    OH−>F−>NH3>CN−\mathrm{OH}^- > \mathrm{F}^- > \mathrm{NH_3} > \mathrm{CN}^-

Correct Answerb

CO>H2O>F−>S2−\mathrm{CO} > \mathrm{H_2O} > \mathrm{F}^- > \mathrm{S}^{2-}

Detailed Solution

image 🧠 The Three-Tier Mental Map

Spectrochemical series problems sound like memory games, but they collapse to a single picture: a three-tier ladder. Bottom rung — heavy/diffuse donors (I−\mathrm{I^-}, S2−\mathrm{S^{2-}}, halides). Middle rung — hard O/N donors (H2O\mathrm{H_2O}, OH−\mathrm{OH^-}, NH3\mathrm{NH_3}, en). Top rung — π\pi-acceptors (CN−\mathrm{CN^-}, CO\mathrm{CO}). Once the rungs are in your head, "decreasing order" is just reading top-to-bottom.

🗺️ Walk the Ladder

Take each option and check whether it walks down the ladder.

(1) NCS−>EDTA4−>CN−>CO\mathrm{NCS^- > EDTA^{4-} > CN^- > CO} — places CO\mathrm{CO} last. CO\mathrm{CO} is the strongest field ligand we routinely meet. Wrong direction.

(2) CO>H2O>F−>S2−\mathrm{CO > H_2O > F^- > S^{2-}} — top rung → middle rung → halide → sulfide. Clean descent across all three tiers.

(3) Starts with S2−\mathrm{S^{2-}} as strongest. S2−\mathrm{S^{2-}} is among the weakest — disqualified instantly.

(4) Places CN−\mathrm{CN^-} last. CN−\mathrm{CN^-} is near the top. Wrong.

⚡ The 5-Second Filter

Whenever the option ranks CO\mathrm{CO} or CN−\mathrm{CN^-} as weakest, or S2−/I−\mathrm{S^{2-}}/\mathrm{I^-} as strongest, kill it without further thought. In four-option MCQs this alone usually leaves one survivor.

⚠️ The Trap That Catches Most

Students confuse "field strength" with "basicity / charge". S2−\mathrm{S^{2-}} has a −2-2 charge, CO\mathrm{CO} is neutral — yet CO\mathrm{CO} wins by a mile. The reason is π\pi-back-donation: empty π∗\pi^* orbitals on CO\mathrm{CO} accept metal d\mathrm{d}-electron density, dramatically widening Δ\Delta. Charge alone does not predict the series.

Answer: (2) CO>H2O>F−>S2−\boxed{\text{Answer: (2) } \mathrm{CO > H_2O > F^- > S^{2-}}}

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