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. 20Crystal Field Theory
8 MCQs2 revision cards9-step worked example
Source: NCERT Coordination CompoundsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
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 20).
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 22).
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.
Practice 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?
MCQ 2Easy RecallPractice
Which of the following is the correct order of ligands in the spectrochemical series (weakest to strongest field)?
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:
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:
MCQ 5Direct ApplicationPractice
[CoF₆]³⁻ contains Co³⁺ (d⁶). The magnetic moment of this complex is closest to:
MCQ 6Direct ApplicationPractice
Which of the following tetrahedral complexes is most likely to be low-spin?
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:
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:
Quick recall before you leave
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. ---
Before solving, remember these
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. 22Key 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. 23Formulas
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)
Exam Traps & Common Mistakes
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.
Past Year Questions
11 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1C < A < D < B
2B < D < A < C
3B < A < D < C
4C < D < A < B
NTA Answer: Option 3(final)
NEET 2025
1Statement I is false but Statement II is true
2Both Statement I and Statement II are true
3Both Statement I and Statement II are false
4Statement I is true but Statement II is false
NTA Answer: Option 4(final)
NEET 2024Revised key
1B and C
2A and D
3B and E
4E and D
NTA Answer: Option 3(revised_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 2023
1A, B, C, D are correct
2A, B, C, E are correct
3A, C, D, E are correct
4B, C, D, E are correct
NTA Answer: Option 2(final)
NEET 2023
Homoleptic complex from the following complexes is
1Diamminechloridonitrito-N-platinum (II)
2Pentaamminecarbonatocobalt (III) chloride
3Triamminetriaquachromium (III) chloride
4Potassium trioxalatoaluminate (III)
NTA Answer: Option 4(final)
NEET 2022
Identify the incorrect statement from the following.
1The shapes of d xy , d yz and d zx orbitals are similar to each other; and d x2−y2 and d z2 are similar to each other.
2All the five 5d orbitals are different in size when compared to the respective 4d orbitals.
3All the five 4d orbitals have shapes similar to the respective 3d orbitals.
4In an atom, all the five 3d orbitals are equal in energy in free state.
NTA Answer: Option 1(final)
NEET 2022
The IUPAC name of the complex- [Ag(H O) ][Ag(CN) ] is: 2 2 2
1diaquasilver(I) dicyanidoargentate(I)
2dicyanidosilver(II) diaquaargentate(II)
3diaquasilver(II) dicyanidoargentate(II)
4dicyanidosilver(I) diaquaargentate(I)
NTA Answer: Option 1(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 2021
12-methylbutan-2-ol
22-methylpropan-2-ol
3pentan-2-ol
4pentan-3-ol
NTA Answer: Option 1(final)
NEET 2021
Ethylene diaminetetraacetate (EDTA) ion is :
1Tridentate ligand with three "N" donor atoms
2Hexadentate ligand with four "O" and two "N" donor atoms
3Unidentate ligand
4Bidentate ligand with two "N" donor atoms
NTA Answer: Option 2(final)
How NEET usually asks this
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
Apply IUPAC rules: ligands alphabetical, prefixes for multiplicity, oxidation state of metal.
RecallMedium
Common distractors
wrong order ligands
Uses non-alphabetical order
Identify primary and secondary valencies, coordination number from formula. Primary = ionizable ions; secondary = ligands.
InterpretationMedium
Common distractors
counts counter ions as ligands
Treats all attached species as ligands
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.20 | Class 12 Chemistry Chapter 5, p.22
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