Coordination Chemistry Magnetic Moment Order Question JEE Main 2026

Quick Summary

Question Type: Spin-only magnetic moment comparison of coordination compounds
Chapter: Coordination Chemistry – Magnetic Properties of Complexes
Difficulty: ⭐⭐ Easy
Time to Solve: 3-4 minutes
Key Formula: μ = √[n(n + 2)] BM
Correct Answer: (C) C < B < D < A
Why: [Ni(CN)₄]²⁻ is diamagnetic, [Cu(H₂O)₆]²⁺ has 1 unpaired electron, [Ni(H₂O)₆]²⁺ has 2 unpaired electrons, and [MnBr₄]²⁻ has 5 unpaired electrons. Therefore, the increasing order is C < B < D < A.

The Question

JEE Main 2026 (28 January Evening Shift) – Coordination Chemistry

The correct increasing order of spin-only magnetic moment values of the complex ions

[MnBr₄]^2- (A), [Cu(H₂O)₆]²⁺ (B), [Ni(CN)₄]^2- (C) and [Ni(H₂O)₆]²⁺ (D) is:

(A) A = B < D < C
(B) C = D < B < A
(C) C < B < D < A
(D) A = B < C < D

Quick Answer

Correct Option: (C) C < B < D < A

Reasoning: Spin-only magnetic moment depends on the number of unpaired electrons.

• [Ni(CN)₄]^2- is square planar and diamagnetic → 0 unpaired electrons
• [Cu(H₂O)₆]²⁺ has 1 unpaired electron
• [Ni(H₂O)₆]²⁺ has 2 unpaired electrons
• [MnBr₄]^2- has 5 unpaired electrons

So the increasing order of magnetic moment is:

C < B < D < A

Video Solution

If you want the full explanation in a clear step-by-step teaching format, watch the video solution below:

Watch Full Video Solution on YouTube

Understanding the Concept

Why Magnetic Moment Depends on Unpaired Electrons

The spin-only magnetic moment of a coordination compound depends only on the number of unpaired electrons present in the d-orbitals of the metal ion.

μ = √[n(n + 2)] BM

Where:

• μ = magnetic moment in Bohr Magneton (BM)
• n = number of unpaired electrons

So to compare magnetic moments, we only need to find the number of unpaired electrons in each complex.

The Key Principle

1. Find oxidation state of the metal ion
2. Write its d-electron configuration
3. Check ligand strength and geometry
4. Count unpaired electrons
5. More unpaired electrons means higher magnetic moment

Detailed Step-by-Step Solution

Step 1: Recall the Spin-Only Formula

• μ = √[n(n + 2)] BM
• Magnetic moment increases with the number of unpaired electrons
• So we compare complexes by counting unpaired electrons

Step 2: Analyze Each Complex

Download Handwritten Solution

(A) [MnBr₄]^2-

• Oxidation state of Mn = +2
• Mn²⁺ = 3d⁵
• Br⁻ is a weak field ligand
• Tetrahedral complex remains high spin
• Unpaired electrons = 5

(B) [Cu(H₂O)₆]²⁺

• Oxidation state of Cu = +2
• Cu²⁺ = 3d⁹
• Unpaired electrons = 1

(C) [Ni(CN)₄]^2-

• Oxidation state of Ni = +2
• Ni²⁺ = 3d⁸
• CN⁻ is a strong field ligand
• Complex becomes square planar
• All electrons are paired
• Unpaired electrons = 0

(D) [Ni(H₂O)₆]²⁺

• Oxidation state of Ni = +2
• Ni²⁺ = 3d⁸
• H₂O is a weak field ligand
• Octahedral high-spin arrangement
• Unpaired electrons = 2

Step 3: Compare Number of Unpaired Electrons

• C → 0 unpaired electrons
• B → 1 unpaired electron
• D → 2 unpaired electrons
• A → 5 unpaired electrons

Step 4: Write the Magnetic Moment Order

Since magnetic moment increases with unpaired electrons:

C < B < D < A

Final Answer

Option (C): C < B < D < A ✓

The correct increasing order of spin-only magnetic moment values is [Ni(CN)₄]^2- < [Cu(H₂O)₆]²⁺ < [Ni(H₂O)₆]²⁺ < [MnBr₄]^2-.

Essential Formulas for This Topic

Primary Equations

1. Spin-Only Magnetic Moment:
   • μ = √[n(n + 2)] BM
   • n = number of unpaired electrons
   • Used for quick JEE magnetic property questions
2. d-Electron Counting Rule:
   • Find oxidation state first
   • Then determine d-electron configuration of the metal ion
   • Ligand strength affects pairing
3. Ligand Strength Logic:
   • CN⁻ is strong field
   • H₂O and Br⁻ are weak field ligands
   • Strong field ligands promote pairing

Important Constants

• More unpaired electrons means larger magnetic moment
• Square planar d⁸ complexes are often diamagnetic
• Tetrahedral complexes are generally high spin
• Weak field ligands usually do not cause pairing

Common Mistakes to Avoid

❌ Mistake 1: Ignoring Ligand Strength

Wrong Thinking: “All Ni²⁺ complexes must have the same magnetic behavior.”

Correct Approach: Ligand strength changes electron pairing. [Ni(CN)₄]^2- and [Ni(H₂O)₆]²⁺ behave differently because CN⁻ is strong field and H₂O is weak field.

❌ Mistake 2: Forgetting Geometry

Wrong Approach: Not checking whether the complex is tetrahedral, octahedral, or square planar

Correct Approach: Geometry strongly affects splitting and pairing, especially in d⁸ complexes.

❌ Mistake 3: Miscounting d-Electrons

Common Error:

• Taking Ni²⁺ as d⁶ or Cu²⁺ as d⁸
• Ignoring oxidation state before writing configuration

Correct Approach: Always determine oxidation state first, then write the correct d-electron count.

❌ Mistake 4: Thinking Diamagnetic Means Small but Nonzero Moment

Wrong Thinking: “Diamagnetic complexes still have some spin-only moment.”

Correct Understanding:

• Diamagnetic means all electrons are paired
• So n = 0
• Hence spin-only magnetic moment is zero
• That is why C is the lowest in the order

Key Concept Summary

What You Must Remember

1. Magnetic moment depends on unpaired electrons: More unpaired electrons means larger μ
2. Find oxidation state first: Then write d-electron configuration
3. Ligand strength matters: CN⁻ pairs electrons, H₂O and Br⁻ usually do not
4. Geometry matters too: Square planar d⁸ is often diamagnetic
5. Count carefully: C has 0, B has 1, D has 2, A has 5 unpaired electrons

The Golden Rule for Magnetic Moment Questions

“Find oxidation state, d-electron count, ligand strength, and geometry before comparing unpaired electrons.”

Frequently Asked Questions

Q1: Why is [Ni(CN)₄]^2- diamagnetic?

A: Because CN⁻ is a strong field ligand and the complex becomes square planar, causing all electrons to pair.

Q2: Why does [MnBr₄]^2- have the highest magnetic moment?

A: Mn²⁺ is d⁵ and Br⁻ is a weak field ligand, so all five electrons remain unpaired.

Q3: Why is [Cu(H₂O)₆]²⁺ not diamagnetic?

A: Cu²⁺ is d⁹, so one electron remains unpaired.

Q4: Why is [Ni(H₂O)₆]²⁺ more magnetic than [Cu(H₂O)₆]²⁺?

A: Because [Ni(H₂O)₆]²⁺ has 2 unpaired electrons, while [Cu(H₂O)₆]²⁺ has only 1.

Q5: What is the final increasing order of magnetic moment?

A: C < B < D < A.

Prerequisites to Solve This Question

Before attempting this problem, you should understand:

1. Oxidation state: Determining metal ion charge in complexes
2. d-Block configurations: Writing d-electron count of metal ions
3. Crystal field theory basics: Strong field and weak field ligands
4. Coordination geometry: Tetrahedral, octahedral and square planar complexes
5. Magnetic behavior: Difference between paramagnetic and diamagnetic complexes

After Solving This, You Can:

✅ Compare spin-only magnetic moments confidently
✅ Count unpaired electrons in coordination compounds
✅ Use ligand strength to predict pairing
✅ Distinguish diamagnetic and paramagnetic complexes
✅ Solve JEE Main coordination compound magnetic questions faster
✅ Apply the same logic to more advanced crystal field problems

Study Tips for This Topic

For JEE Main:

1. Memorize strong and weak field ligands: This helps immediately in pairing decisions
2. Practice oxidation state calculation: Wrong oxidation state ruins the whole question
3. Remember common special cases: Square planar d⁸ complexes are very important
4. Use unpaired electron count directly: It is enough for spin-only moment order questions

Common JEE Variants:

• Increasing or decreasing magnetic moment order
• Find diamagnetic complex
• Find number of unpaired electrons
• Compare strong field and weak field complexes

Difficulty Rating & Exam Frequency

Difficulty Level: ⭐⭐ (2/5) – Easy
JEE Main Frequency: High – Magnetic property questions are common in Coordination Chemistry
JEE Advanced Frequency: Medium – Often combined with CFT and geometry concepts
Topic Importance: Very High – Magnetic behavior is a core part of coordination chemistry

Written by Nishant Kumar
Chemistry Educator with 10+ Years of Experience Teaching JEE Aspirants
Founder – PadhoLikhoJEE

Last Updated: March 2026
Question Source: JEE Main 2026 PYQ
Topic: Coordination Chemistry – Magnetic Properties of Complexes
“`