Ligands treated as point charges/dipoles split d-orbitals. Octahedral splitting: t₂g (lower, 3 orbitals) and e_g (upper, 2 orbitals); difference Δ_o. Tetrahedral: opposite splitting; Δ_t = (4/9) Δ_o.
-- NCERT Class 12 Chemistry, Ch. 5, p. 131Crystal Field Theory
8 MCQs2 revision cards9-step worked example
Source: NCERT Coordination CompoundsPYQ coverage: NEET 2020, 2022, 2024, 2025, 2026Official key: NTA-verifiedLast updated: 26 Sep 2026
Crystal Field Theory, explained for NEET
The trap that costs marks here: you see a coordination compound, identify the metal's d-electron count correctly, then assign high-spin or low-spin by gut feeling instead of checking the ligand's position in the spectrochemical series. The result: wrong number of unpaired electrons → wrong magnetic moment → wrong answer. This is the single most tested confusion in CFT questions (observed NEET 2021, 2024).
Crystal field theory in 90 seconds. When ligands approach a transition metal ion, the five d-orbitals lose their degeneracy. In an octahedral field, the d-orbitals split into two sets: t₂g (lower energy, 3 orbitals) and eg (higher energy, 2 orbitals). The energy gap between them is Δo (NCERT Class 12 Chemistry Chapter 5, page 132).
The decision that matters: whether electrons pair in t₂g or jump to eg depends on the comparison between Δo and the pairing energy P.
- Strong-field ligand (CN⁻, CO, NO₂⁻, NH₃): large Δo → Δo > P → electrons pair in t₂g → low-spin.
- Weak-field ligand (I⁻, Br⁻, Cl⁻, F⁻, H₂O): small Δo → Δo < P → electrons occupy eg before pairing → high-spin.
Tetrahedral splitting is always smaller: Δt ≈ (4/9)Δo. Because Δt is small, tetrahedral complexes are almost always high-spin regardless of ligand (NCERT Class 12 Chemistry Chapter 5, page 139).
Spectrochemical series (commit to memory): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO
Watch-out: NH₃ is strong-field for most 3d metals, but H₂O is weak-field. Students frequently swap these two.
Can you answer these Crystal Field Theory MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
In crystal field theory, the d-orbitals in an octahedral complex split into which two sets?
Show answer and why every option is right or wrong
Answer: D. In an octahedral crystal field, the three orbitals directed between the axes (dxy, dyz, dxz) form the lower-energy t₂g set, and the two orbitals directed along the axes (dx²−y², dz²) form the higher-energy eg set (NCERT Class 12 Chemistry Chapter 5, page 131).
Why A is wrong: Crystal field theory specifically explains the lifting of degeneracy — an octahedral field always splits d-orbitals into t₂g and eg.
Why B is wrong: This reverses the t₂g and eg labels — eg orbitals point along the axes (toward ligands) and have higher energy, not lower.
Why C is wrong: This incorrectly groups dx²−y² with t₂g. The dx²−y² orbital points along x and y axes directly at ligands, placing it in the higher-energy eg set.
MCQ 2Easy RecallPractice
Which of the following is the correct order of ligands in the spectrochemical series (weakest to strongest field)?
Show answer and why every option is right or wrong
Answer: B. The spectrochemical series places halides as weak-field, with Cl⁻ < F⁻, then H₂O < NH₃ < CN⁻ in increasing field strength (NCERT Class 12 Chemistry Chapter 5, page 132).
Why A is wrong: This is the reverse order. CN⁻ and CO are the strongest-field ligands, not the weakest.
Why C is wrong: This places F⁻ before Cl⁻ (wrong — Cl⁻ is weaker than F⁻) and puts CN⁻ below NH₃ (wrong — CN⁻ is stronger than NH₃).
Why D is wrong: This scrambles the order entirely — H₂O is not weaker than F⁻ in the spectrochemical series, and Cl⁻ is not stronger than CN⁻.
MCQ 3Easy RecallPractice
The relationship between tetrahedral crystal field splitting (Δt) and octahedral crystal field splitting (Δo) for the same metal-ligand combination is approximately:
Show answer and why every option is right or wrong
Answer: A. Tetrahedral splitting is approximately 4/9 of octahedral splitting because fewer ligands (4 vs 6) interact with d-orbitals and they are not directed along the orbital axes (NCERT Class 12 Chemistry Chapter 5, page 139).
Why B is wrong: This would mean tetrahedral and octahedral splitting are equal, which contradicts the geometric difference — tetrahedral has fewer ligands at non-axial positions.
Why C is wrong: This is the inverse ratio (9/4). This would make tetrahedral splitting larger than octahedral, which contradicts the fact that fewer ligands at indirect angles produce a smaller splitting.
Why D is wrong: The 2/3 ratio has no theoretical basis in crystal field theory. The correct derivation gives 4/9.
MCQ 4Direct ApplicationPractice
[Fe(CN)₆]⁴⁻ contains Fe²⁺ (d⁶). Given that CN⁻ is a strong-field ligand, the number of unpaired electrons in this complex is:
Show answer and why every option is right or wrong
Answer: D. CN⁻ is strong-field → Δo > P → low-spin octahedral. For d⁶ low-spin: all 6 electrons pair in t₂g (t₂g⁶ eg⁰), giving 0 unpaired electrons (NCERT Class 12 Chemistry Chapter 5, page 140).
Why A is wrong: 4 unpaired electrons corresponds to high-spin d⁶ (t₂g⁴ eg²). This ignores that CN⁻ is a strong-field ligand causing electron pairing (trap: defaulting to high-spin without checking ligand field strength).
Why B is wrong: 2 unpaired electrons does not correspond to any valid d⁶ filling in either high-spin or low-spin octahedral configuration.
Why C is wrong: 1 unpaired electron does not match any d⁶ configuration in octahedral geometry — it's not possible to get exactly 1 unpaired electron from 6 d-electrons in either spin state.
MCQ 5Direct ApplicationPractice
[CoF₆]³⁻ contains Co³⁺ (d⁶). The magnetic moment of this complex is closest to:
Show answer and why every option is right or wrong
Answer: C. F⁻ is a weak-field ligand → high-spin d⁶ → t₂g⁴ eg² → 4 unpaired electrons. μ = √(4×6) = √24 ≈ 4.90 BM (NCERT Class 12 Chemistry Chapter 5, page 140).
Why A is wrong: 0 BM would mean low-spin d⁶ with all electrons paired. This ignores that F⁻ is a weak-field ligand — Δo < P, so electrons remain unpaired (trap: defaulting to low-spin without checking ligand).
Why B is wrong: 5.92 BM corresponds to 5 unpaired electrons (√35), which is impossible for a d⁶ configuration — maximum unpaired electrons for d⁶ is 4.
Why D is wrong: 3.87 BM corresponds to 3 unpaired electrons (√15). For d⁶ in octahedral geometry, you get either 4 (high-spin) or 0 (low-spin) unpaired electrons, never 3.
MCQ 6Direct ApplicationPractice
Which of the following tetrahedral complexes is most likely to be low-spin?
Show answer and why every option is right or wrong
Answer: B. Tetrahedral crystal field splitting (Δt ≈ 4/9 Δo) is too small to overcome pairing energy for common ligands and metals, so tetrahedral complexes are almost always high-spin (NCERT Class 12 Chemistry Chapter 5, page 139).
Why A is wrong: [NiCl₄]²⁻ is tetrahedral with a weak-field ligand (Cl⁻). Even with a strong-field ligand, tetrahedral Δt would be too small for pairing.
Why C is wrong: [FeCl₄]²⁻ is tetrahedral with Cl⁻. There is no realistic scenario where this complex becomes low-spin given the inherently small tetrahedral splitting.
Why D is wrong: [CoCl₄]²⁻ is tetrahedral with Cl⁻ (weak-field). The small Δt means electrons will not pair regardless of the metal.
MCQ 7CalculationPractice
A student is given two octahedral complexes of Mn²⁺ (d⁵): complex X with H₂O as ligand and complex Y with CN⁻ as ligand. The difference in their magnetic moments (μX − μY) is closest to:
Show answer and why every option is right or wrong
Answer: C. Mn²⁺ is d⁵. H₂O is weak-field → high-spin → 5 unpaired → μX = √(5×7) = √35 ≈ 5.92 BM. CN⁻ is strong-field → low-spin → 1 unpaired (t₂g⁵) → μY = √(1×3) = √3 ≈ 1.73 BM. Difference = 5.92 − 1.73 ≈ 4.18 BM.
Why A is wrong: 5.92 BM is the magnetic moment of high-spin d⁵ itself, not the difference between the two complexes. The CN⁻ complex is low-spin with μ = 1.73 BM, so the difference is smaller.
Why B is wrong: 1.73 BM is the magnetic moment of the low-spin complex Y alone, not the difference between X and Y.
Why D is wrong: 0 BM difference would mean both complexes have the same number of unpaired electrons. This ignores that H₂O (weak-field) and CN⁻ (strong-field) produce different spin states for d⁵ (trap: defaulting to same spin state regardless of ligand).
MCQ 8CalculationPractice
An octahedral complex [M(NH₃)₆]³⁺ has a magnetic moment of 2.83 BM. The number of unpaired electrons and the most likely metal ion are:
Show answer and why every option is right or wrong
Answer: A. μ = √(n(n+2)) = 2.83 BM → n(n+2) = 8 → n = 2 unpaired electrons. NH₃ is a strong-field ligand. V³⁺ is d² → in strong-field octahedral: t₂g² eg⁰ → 2 unpaired electrons. This matches (NCERT Class 12 Chemistry Chapter 5, page 140).
Why B is wrong: Ni²⁺ (d⁸) in octahedral strong-field gives t₂g⁶ eg² → also 2 unpaired, but the complex formula states M³⁺ charge, and Ni²⁺ doesn't match the 3+ oxidation state in [M(NH₃)₆]³⁺.
Why C is wrong: 1 unpaired electron gives μ = √3 ≈ 1.73 BM, not 2.83 BM. The calculation doesn't match.
Why D is wrong: 3 unpaired electrons gives μ = √15 ≈ 3.87 BM, not 2.83 BM. Cr³⁺ (d³) with NH₃ gives t₂g³ with 3 unpaired electrons — the magnetic moment would be higher than observed.
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Crystal Field Theory: quick recall before you leave
How do you solve a Crystal Field Theory question? A worked example
Pattern: Determine high-spin or low-spin from ligand field strength (P.CHE.U12.CRYSTAL_FIELD_HIGH_LOW_SPIN)
- 1
Given
• Complex: [Fe(H₂O)₆]²⁺ (octahedral)• Metal ion: Fe²⁺• Electron configuration: d⁶• Ligand: H₂O
- 2
Required
• Number of unpaired electrons• Magnetic moment (in BM)
- 3
Concept
Crystal field theory: in an octahedral field, d-orbitals split into t₂g (lower) and eg (higher). The spin state depends on the relative magnitude of Δo vs pairing energy P. The spectrochemical series tells us the field strength of H₂O.
- 4
Formula
μ = √(n(n+2)) BM
- 5
Substitution logic
• H₂O is a weak-field ligand (spectrochemical series: below NH₃, en, CN⁻)• Weak-field → Δo < P → high-spin• d⁶ high-spin filling: t₂g⁴ eg² (fill one electron in each of the 5 orbitals first, then the 6th pairs in t₂g)• Unpaired electrons: 4
- 6
Calculation
μ = √(4 × (4+2)) = √(4 × 6) = √24 ≈ 4.90 BM
Note: The integers 4 and 6 in n(n+2) are exact counting numbers derived from electron count — they do not limit significant figures. - 7
Final answer
μ ≈ 4.90 BM (paramagnetic, 4 unpaired electrons)
- 8
Common trap
If you default to low-spin without checking the ligand, you'd assign t₂g⁶ eg⁰ → 0 unpaired → μ = 0 (diamagnetic). This is wrong because H₂O is a weak-field ligand. The correct answer requires recognising H₂O's position in the spectrochemical series.
- 9
Similar NEET-style question
Calculate the spin-only magnetic moment of [Mn(CN)₆]⁴⁻. (Mn²⁺ is d⁵; CN⁻ is strong-field.) Answer: low-spin d⁵ → t₂g⁵ → 1 unpaired → μ = √3 ≈ 1.73 BM.
---
What to remember before solving Crystal Field Theory questions
Key Fact
High vs low spin
If Δ > P (pairing energy): low spin (inner orbital, e⁻ pair up). If Δ < P: high spin (outer orbital). Strong-field ligands (CN⁻, CO, NH₃ for some metals): low spin. Weak field (F⁻, H₂O, OH⁻): high spin.
-- NCERT Class 12 Chemistry, Ch. 5, p. 132Key Fact
Spectrochemical series
Ligands ordered by increasing Δ: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO. Predicts colour and magnetic properties of complexes.
-- NCERT Class 12 Chemistry, Ch. 5, p. 132Which Crystal Field Theory formulas do you need for NEET?
Crystal field splitting (octahedral vs tetrahedral)
Tetrahedral splitting is smaller than octahedral due to fewer/farther ligands.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Delta_o | octahedral splitting | J or eV |
| Delta_t | tetrahedral splitting | J or eV |
Valid when
- Same metal and same ligand
- Mostly high-spin tetrahedral due to small Δ_t
Magnetic moment of coordination complex
Same spin-only formula but n depends on high-spin/low-spin from CFT.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | unpaired electrons | - |
Valid when
- High vs low spin determined by Δ_o vs P
- Octahedral (or tetrahedral with Δ_t)
Where do students lose marks on Crystal Field Theory?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student defaults to one spin state. Strong-field ligand (CN⁻, CO, NH₃ for some) → low-spin (Δ > P, electrons pair). Weak-field (F⁻, H₂O, Cl⁻) → high-spin.
When it triggers
Coordination compound with given ligand asking for magnetic moment, color, or spin state.
How to avoid
Memorise spectrochemical series: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ < CO. NH₃, CN⁻, CO usually strong-field. F⁻, H₂O, Cl⁻ usually weak-field.
Root cause: concept gap
Correction
Strong-field ligand (CN⁻, CO, NH₃ when applicable): Δ > P → low-spin. Weak-field (F⁻, Cl⁻, H₂O): Δ < P → high-spin. Use spectrochemical series.
More in Coordination Compounds: 2 question patterns from its other lessons.
Crystal Field Theory questions from past NEET papers
5 questions from NEET 2020, 2022, 2024, 2025, 2026. Answers verified against NTA official keys.
NEET 2026
1CO > NH₃ > H₂O > Cl⁻
2CO > H₂O > NH₃ > Cl⁻
3Cl⁻ > H₂O > NH₃ > CO
4Cl⁻ > NH₃ > H₂O > CO
NTA Answer: Option 1(final)
NEET 2025
1C < A < D < B
2B < D < A < C
3B < A < D < C
4C < D < A < B
NTA Answer: Option 3(final)
NEET 2024Revised key
1Both Statement I and Statement II are true
2Both Statement I and Statement II are false
3Statement I is true but Statement II is false
4Statement I is false but Statement II is true
NTA Answer: Option 1(revised_final)
NEET 2022
1(B) > (A) > (C)
2(A) > (B) > (C)
3(C) > (B) > (A)
4(C) > (A) > (B)
NTA Answer: Option 4(final)
NEET 2020
1CN⁻ < C₂O₄²⁻ < SCN⁻ < F⁻
2SCN⁻ < F⁻ < C₂O₄²⁻ < CN⁻
3SCN⁻ < F⁻ < CN⁻ < C₂O₄²⁻
4F⁻ < SCN⁻ < C₂O₄²⁻ < CN⁻
NTA Answer: Option 2(final)
How does NEET ask about Crystal Field Theory?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Determine high-spin or low-spin from ligand field strength. Strong-field (CN, CO): low-spin. Weak-field (F, H2O): high-spin.
InterpretationMedium
Common distractors
forgets weak vs strong ligand
Default low-spin always
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.132 | Class 12 Chemistry Chapter 5, p.139
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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