Q.81 Chemistry · Section A ⭐ 360° Option Elimination Active

NEET 2024 Chemistry Question 81 Solution

Topic: Coordination Compounds · Official NTA Booklet Code T1

Question

Given below are two statements: Statement I: Both [Co(NH₃)₆]³⁺ and [CoF₆]³⁻ complexes are octahedral but differ in their magnetic behaviour. Statement II: [Co(NH₃)₆]³⁺ is diamagnetic whereas [CoF₆]³⁻ is paramagnetic. In the light of the above statements, choose the correct answer from the options given below:

✓

Verified Against NTA Final Answer Key

Option (3) : Both Statement I and Statement II are true.

📚 Coordination Compounds Easy

📋 Given Parameters & Problem Setup

Complex 1 [Co(NH3)6]³⁺ (hexaamminecobalt(III) ion)

Complex 2 [CoF6]³⁻ (hexafluoridocobaltate(III) ion)

Statement I Both complexes are octahedral but differ in their magnetic behaviour

Statement II [Co(NH3)6]³⁺ is diamagnetic whereas [CoF6]³⁻ is paramagnetic

⚡

The 30-Second Exam Hall Shortcut: ⚡ Strong Field vs Weak Field Spectrochemical Hack

⏱️ Save ~40 seconds

Co³⁺ is d⁶. NH3 is a strong field ligand -> Δ_o > P -> pairing -> t_2g⁶ e_g⁰ (n=0, diamagnetic)! F⁻ is a weak field ligand -> Δ_o < P -> t_2g⁴ e_g² (n=4, paramagnetic)!

1 Both complexes have coordination number 6 -> Both are Octahedral geometry -> Statement I part 1 is TRUE.

2 Oxidation state of Co is +3 in both: Co³⁺ = [Ar] 3d⁶.

3 In [Co(NH3)6]³⁺, NH3 is a strong field ligand. Crystal field splitting Δ_o exceeds pairing energy P (Δ_o > P), causing complete pairing into t_2g⁶ e_g⁰ -> n = 0 unpaired electrons -> DIAMAGNETIC.

4 In [CoF6]³⁻, F⁻ is a weak field ligand (Δ_o < P), leading to high-spin configuration t_2g⁴ e_g² -> n = 4 unpaired electrons -> PARAMAGNETIC.

5 Therefore, both are octahedral, differ in magnetic behavior (Statement I is TRUE), and [Co(NH3)6]³⁺ is diamagnetic while [CoF6]³⁻ is paramagnetic (Statement II is TRUE).

6 Both statements are true -> Option (3) in 5 seconds!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Coordination Compounds (Crystal field theory in octahedral complexes: [Co(NH3)6]3+ diamagnetic low-spin vs [CoF6]3- paramagnetic high-spin).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Coordination Number and Geometry

Both [Co(NH3)6]³⁺ and [CoF6]³⁻ possess a central cobalt ion surrounded by six monodentate ligands (coordination number = 6). Coordination number 6 uniquely dictates an Octahedral coordination polyhedral geometry for both complexes:

\text{CN} = 6 \implies \text{Octahedral Geometry for both complexes}

Step 2

Crystal Field Analysis of [Co(NH3)6]³⁺

Cobalt(III) has a 3d⁶ valence electron configuration. NH3 occupies a high position in the spectrochemical series and behaves as a strong field ligand with Co³⁺. Because Δo > P (pairing energy), electrons fully pair in the lower energy t2g orbitals:

\text{Configuration: } t_{2g}^6\,e_g^0, \quad n = 0 \implies \text{Diamagnetic (Low-spin / inner orbital complex)}

Step 3

Crystal Field Analysis of [CoF6]³⁻

Fluoride ion (F⁻) lies at the weak field end of the spectrochemical series. The crystal field splitting is smaller than the pairing energy (Δo < P), so electrons follow Hund’s rule of maximum multiplicity across both t2g and eg sets:

\text{Configuration: } t_{2g}^4\,e_g^2, \quad n = 4 \implies \mu = \sqrt{4(6)} = 4.90\;\text{B.M.} \; (\text{Paramagnetic / high-spin})

Step 4

Evaluating Statements

Statement I correctly asserts that both are octahedral but differ in magnetic behavior. Statement II correctly specifies that [Co(NH3)6]³⁺ is diamagnetic and [CoF6]³⁻ is paramagnetic. Both statements are factually and theoretically correct.

\text{Both Statement I and Statement II are true (Option 3)}

⭐

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 (1)

✗ Incorrect

Verdict & Scientific Reason:

Statement II is the precise textbook statement of their magnetic properties.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking [CoF6]3- is diamagnetic.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Statement I is true: both complexes are octahedral.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking they have different geometries.

3 (3)

✓ 100% Correct

Verdict & Scientific Reason:

Both statements reflect canonical crystal field theory and empirical magnetic measurements.

💡

Why this is airtight: None.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Both statements are completely verified truths.

⚠️

Student Fallacy (Why candidates pick this trap): Total rejection of CFT.

⚠️

NTA Trap Alert: Assuming All Co(III) Complexes Have Identical Magnetism

Students frequently remember Co³⁺ as diamagnetic from [Co(NH3)6]³⁺, forgetting that weak field ligands like F⁻ produce high-spin paramagnetic complexes (n = 4).

🛡️ How to avoid the -1 mark: Always check ligand strength: NH3 = strong (diamagnetic); F⁻ = weak (paramagnetic).

📖

Core NEET Syllabus Standard

Chemistry Core Inorganic Chemistry · Coordination Compounds Standard Benchmark

Core Scientific Principle Tested:

"In octahedral field, d6 forms diamagnetic t2g⁶ eg⁰ with strong field ligands (NH3) and paramagnetic t2g⁴ eg² with weak field ligands (F⁻)."

Syllabus Focus: Crystal Field Theory: Electronic Configurations of d6 Ions

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ammonia pairs them all (diamagnetic zero); Fluoride spreads them wide (paramagnetic hero)!

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NEET 2024 Chemistry Question 81 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Strong Field vs Weak Field Spectrochemical Hack

Step-by-Step Rigorous Workflow & Mathematical Proof

Coordination Number and Geometry

Crystal Field Analysis of [Co(NH3)6]³⁺

Crystal Field Analysis of [CoF6]³⁻

Evaluating Statements

360° Option Elimination Engine Key Feature

NTA Trap Alert: Assuming All Co(III) Complexes Have Identical Magnetism

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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