The mismatched combinations are: A. Chlorophyll – Co B. Water hardness – EDTA C. Photography – [Ag(CN)2]- D. Wilkinson…

🧠 Two Pillars of Bio/Industrial Chemistry

This question tests two facts that JEE recycles every year:

1. Chlorophyll — central metal is $\mathrm{Mg}$, not $\mathrm{Co}$. Cobalt belongs to vitamin B₁₂. The Mg/Co swap is the textbook distractor.
2. Photographic fixing — uses sodium thiosulfate ($\mathrm{Na_2S_2O_3}$, "hypo"), forming $[\mathrm{Ag(S_2O_3)_2}]^{3-}$. Not a cyanide complex.

So statements A and C are wrong. Statements B, D, E are textbook-correct.

🗺️ Verify Each Pair

• A. Chlorophyll – Co. Wrong metal. Should be Mg. ✗
• B. Water hardness – EDTA. EDTA titration is the standard method. ✓
• C. Photography – $[\mathrm{Ag(CN)_2}]^-$. Cyanide is used in silver extraction (MacArthur–Forrest), not photo-fixing. ✗
• D. Wilkinson catalyst – $[(\mathrm{Ph_3P})_3\mathrm{RhCl}]$. Definition. ✓
• E. Chelating ligand – D-Penicillamine. Penicillamine chelates copper in Wilson's disease. ✓

Mismatches: A and C only.

⚡ The "Magnesium Heart" Mnemonic

Anchor the bio-metal centres once and never miss them again:

• Chlorophyll → Mg (green plant pigment)
• Haemoglobin → Fe
• Vitamin B₁₂ → Co
• Carbonic anhydrase → Zn

If a question pairs chlorophyll with anything other than Mg, kill the option.

⚠️ The Cyanide-Photography Bait

$[\mathrm{Ag(CN)_2}]^-$ is a real complex — it's used for electroplating silver and to extract silver from ores. Photography uses thiosulfate. The two are easy to mix up because both involve dissolving Ag.

$\boxed{\text{Answer: (2) A and C only}}$