Q.78 Chemistry ⭐ 360° Option Elimination Active

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NEET 2026 Chemistry Question 78 Solution

Topic: Coordination Compounds · Official NTA Booklet Code 50

Question

The complex which has facial and meridional isomers is (Given: py = pyridine and en = H₂N−CH₂−CH₂−NH₂)

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Verified Against NTA Final Answer Key

Option (1) : [Cr(py)₃(Cl)₃]

📚 Coordination Compounds Easy

📋 Given Parameters & Problem Setup

Topic Facial (fac) and meridional (mer) isomerism

Formula Type Required Octahedral complex of type [M a₃ b₃]

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The 30-Second Exam Hall Shortcut: ⚡ The [M a₃ b₃] Fingerprint

⏱️ Save ~45 seconds

Fac-mer isomerism ONLY occurs in octahedral complexes of the stoichiometry [M a₃ b₃]!

1 Scan formulas for an octahedral complex with three of one monodentate ligand and three of another (3:3 ratio).

2 Option (1): [Cr(py)₃(Cl)₃] -> exactly [M a₃ b₃] where a = py, b = Cl⁻!

3 Option (2): [Cr(H₂O)₆]³⁺ is [M a₆] (no geometrical isomers).

4 Option (3): [Co(NH₃)₄(H₂O)₂]³⁺ is [M a₄ b₂] (exhibits cis-trans, NOT fac-mer).

5 Option (4): [Ni(en)₂(H₂O)₂]²⁺ is [M (AA)₂ b₂] (exhibits cis-trans).

6 Only Option (1) has the [M a₃ b₃] formula -> Selected in 5 seconds!

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Why You Can 100% Trust This Shortcut: NCERT Coordination Compounds verbatim: Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma₃b₃] like [Co(NH₃)₃(NO₂)₃].

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Define Facial (fac) and Meridional (mer) Isomerism

In an octahedral complex of type [M a₃ b₃] containing two sets of three unidentate ligands: 1. Facial (fac) isomer: Three donor atoms of the same ligand occupy adjacent positions at the three corners of an octahedral triangular face. 2. Meridional (mer) isomer: Three donor atoms of the same ligand occupy positions around the meridian of the octahedron (two are trans to each other and one is cis).

[\text{M a}_3 \text{b}_3] \xrightleftharpoons{\text{Isomerism}} \text{fac-isomer} + \text{mer-isomer}

• Fac-mer isomerism is unique to the [M a₃ b₃] stoichiometry.

Step 2

Evaluate the Given Complex [Cr(py)₃(Cl)₃]

Pyridine (py) is a neutral monodentate ligand (a). Chloride (Cl⁻) is a uninegative monodentate ligand (b). The complex has the general formula [M a₃ b₃] with chromium in the +3 oxidation state and coordination number 6.

[\text{Cr}(\text{py})_3(\text{Cl})_3] \equiv [\text{M a}_3 \text{b}_3]

• It exists as fac-[Cr(py)₃(Cl)₃] and mer-[Cr(py)₃(Cl)₃].

Step 3

Analyze Other Options

1. [Cr(H₂O)₆]³⁺: Homoleptic [M a₆], no geometrical isomerism. 2. [Co(NH₃)₄(H₂O)₂]³⁺: [M a₄ b₂], exhibits cis-trans isomerism only. 3. [Ni(en)₂(H₂O)₂]²⁺: [M (AA)₂ b₂], exhibits cis-trans isomerism only.

\text{Only } [\text{Cr}(\text{py})_3(\text{Cl})_3] \text{ matches } [\text{M a}_3 \text{b}_3]

• Matches Option (1).

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✓ 100% Correct

Verdict & Scientific Reason:

Exact [M a₃ b₃] formulation displaying both fac and mer geometrical isomers.

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Why this is airtight: None; textbook precision.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Homoleptic complex with 6 identical ligands; has zero geometrical isomers.

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Student Fallacy (Why candidates pick this trap): Assuming any octahedral complex shows geometrical isomerism.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. [M a₄ b₂] complex exhibits cis-trans isomerism, not fac-mer.

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Student Fallacy (Why candidates pick this trap): Confusing cis-trans isomerism of [M a₄ b₂] with fac-mer of [M a₃ b₃].

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Bidentate complex [M (AA)₂ b₂] exhibits cis-trans and optical isomerism, not fac-mer.

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Student Fallacy (Why candidates pick this trap): Mistaking bidentate en coordination for a 3:3 ligand set.

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NTA Trap Alert: Confusing [M a₄ b₂] (cis-trans) with [M a₃ b₃] (fac-mer)

Students frequently remember that octahedral complexes have isomers, but mix up the numbers: 4:2 is cis-trans, whereas 3:3 is fac-mer.

🛡️ How to avoid the -1 mark: 3 and 3 makes FAC and MER! [M a₃ b₃] = facial and meridional.

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Core NEET Syllabus Standard

NEET Chemistry Core Syllabus · Coordination Compounds Standard Benchmark

Core Scientific Principle Tested:

"Octahedral coordination entities of the type [M a₃ b₃] exhibit facial and meridional isomerism."

Syllabus Focus: Geometrical isomerism in octahedral complexes - fac-mer isomerism

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Three of each on the octahedral chair: Fac on the face, Mer around the square!

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NEET 2026 Chemistry Question 78 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The [M a₃ b₃] Fingerprint

Step-by-Step Rigorous Workflow & Mathematical Proof

Define Facial (fac) and Meridional (mer) Isomerism

Evaluate the Given Complex [Cr(py)₃(Cl)₃]

Analyze Other Options

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing [M a₄ b₂] (cis-trans) with [M a₃ b₃] (fac-mer)

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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