Structural: ionisation, hydrate, linkage, coordination. Stereoisomerism: geometrical (cis/trans), optical (chirality). Geometric isomers in [MA₂B₂] type, [MA₃B₃] type. Optical: octahedral [M(en)₃] type.
-- NCERT Class 12 Chemistry, Ch. 5, p. 12Isomerism Coordination
8 MCQs9-step worked example
Source: NCERT Coordination CompoundsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
Isomerism in coordination compounds is a frequent source of lost marks — not because the concept is hard, but because students confuse the type of isomerism a given pair exhibits.
The core idea: Coordination compounds with the same molecular formula can differ in the arrangement of ligands around the metal (structural isomerism) or in the spatial positioning of ligands (stereoisomerism). NCERT Class 12 Chemistry Chapter 5, page 12 establishes two broad classes:
Structural isomerism includes:
- Ionisation isomerism — same formula, different ions in solution. [Co(NH₃)₅Br]SO₄ gives SO₄²⁻ in solution; [Co(NH₃)₅SO₄]Br gives Br⁻. The trap: students forget to check which ion is inside the coordination sphere versus outside.
- Linkage isomerism — ambidentate ligands (NO₂⁻ can bind through N as nitro or O as nitrito; SCN⁻ through S as thiocyanato or N as isothiocyanato).
- Coordination isomerism — exchange of ligands between cationic and anionic coordination entities in salts like [Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆].
- Solvate (hydrate) isomerism — water as ligand vs lattice water. The classic trio: CrCl₃·6H₂O exists as [Cr(H₂O)₆]Cl₃, [Cr(H₂O)₅Cl]Cl₂·H₂O, and [Cr(H₂O)₄Cl₂]Cl·2H₂O.
Stereoisomerism includes:
- Geometrical (cis-trans) isomerism — in square planar [Ma₂b₂] and octahedral [Ma₂b₄] type complexes. Cis = identical ligands adjacent; trans = opposite.
- Optical isomerism — non-superimposable mirror images. Octahedral complexes with bidentate ligands like [Co(en)₃]³⁺ show optical activity.
Watch out: When a question gives a molecular formula and asks "how many isomers?", first classify which types of isomerism apply. Don't jump to drawing geometrical isomers if ionisation isomers also exist — you'll undercount.
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
Which type of isomerism is shown by [Co(NH₃)₅(NO₂)]²⁺ and [Co(NH₃)₅(ONO)]²⁺?
MCQ 2Easy RecallPractice
[Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br are examples of which type of isomerism?
MCQ 3Easy RecallPractice
The compound CrCl₃·6H₂O exists in three hydrate isomeric forms. Which of the following is NOT one of them?
MCQ 4Direct ApplicationPractice
The square planar complex [Pt(NH₃)₂Cl₂] can exhibit geometrical isomerism. How many geometrical isomers does it have, and what are they?
MCQ 5Direct ApplicationPractice
Which of the following octahedral complexes will show geometrical isomerism?
MCQ 6Direct ApplicationPractice
[Co(en)₃]³⁺ exhibits optical isomerism. What structural feature makes this possible?
MCQ 7Concept TrapPractice
A coordination compound has the formula [Co(NH₃)₄Br₂]Cl. A student claims this compound exhibits both geometrical isomerism and ionisation isomerism. Is the student correct?
MCQ 8CalculationPractice
How many total isomers (structural + stereoisomers) are possible for [Co(en)₂Cl₂]⁺?
Worked Example
- 1
Given
- Molecular formula: CoCl₃·4NH₃ - Co is in +3 oxidation state (cobalt(III)), coordination number 6 (octahedral) - This can be written as [Co(NH₃)₄Cl₂]Cl (two Cl⁻ coordinated, one Cl⁻ as counter-ion)
- 2
Required
Total number and types of isomers.
- 3
Concept
We must check for both structural isomerism (ionisation, linkage, etc.) and stereoisomerism (geometrical, optical). Multiple types can coexist for the same formula.
- 4
Identify structural isomers
**Ionisation isomerism:** The Cl⁻ currently outside the coordination sphere can swap with a Cl⁻ inside, but since all three are the same ligand (Cl⁻), exchanging identical ions doesn't produce a new compound. However, if we consider the possibility of [Co(NH₃)₄Cl₂]Cl vs [Co(NH₃)₄Cl(H₂O)]Cl₂ — no, there is no H₂O in the formula. With only NH₃ and Cl⁻, no ionisation isomer exists beyond the [Co(NH₃)₄Cl₂]Cl formulation itself. **Linkage isomerism:** Neither Cl⁻ nor NH₃ is ambidentate. Not applicable.
- 5
Identify stereoisomers
**Geometrical isomerism:** [Co(NH₃)₄Cl₂]⁺ is of type [Ma₄b₂] in octahedral geometry → cis and trans forms exist. **Optical isomerism:** The cis form — does it have a plane of symmetry? In cis-[Co(NH₃)₄Cl₂]⁺, the two Cl⁻ ligands are adjacent. The molecule lacks a plane of symmetry → chiral → d and l forms. The trans form has a plane of symmetry → achiral → no optical isomers.
- 6
Enumerate
1. cis-[Co(NH₃)₄Cl₂]Cl (d-form) 2. cis-[Co(NH₃)₄Cl₂]Cl (l-form) 3. trans-[Co(NH₃)₄Cl₂]Cl
- 7
Final answer
**3 total isomers** — two optical isomers of the cis form plus the trans form.
- 8
Common trap
Students often stop at 2 (cis and trans) and forget to check optical activity of the cis form. Always ask: "Does this geometrical isomer have a plane of symmetry?" If not, it is chiral, and you must count d and l forms separately.
- 9
Similar NEET-style question
"The complex [Cr(en)₂Cl₂]⁺ can exist in how many stereoisomeric forms?" Apply the same two-step analysis: geometrical (cis/trans) → optical check on each → total = 3 (cis-d, cis-l, trans). ---
Before solving, remember these
Formulas
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.12
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