JEE Mains Chemistry · Coordination Compounds
Isomerism in Coordination Compounds
Structural isomers differ in which atoms are bonded to the metal; stereoisomers have the same bonds arranged differently in space, as cis–trans or fac–mer geometrical isomers and as non-superimposable mirror-image optical isomers.
Why this matters
Twenty-three PYQs, fifteen of them multiple choice, and two from 2026. Four name a type of structural isomerism; ten count geometrical isomers or pick the complex that shows cis–trans or fac–mer isomerism; nine count optical isomers or total stereoisomers, usually of an octahedral complex with bidentate ligands.
Concept 1 of 3: Types of structural isomerism in complexes
Structural isomers have the same formula but different bonds. Either a ligand bonds through a different atom (linkage), or a ligand and a counter-ion swap places (ionisation), or ligands swap between a complex cation and a complex anion (coordination), or water moves between the inner sphere and the crystal (solvate). A simple test tells ionisation isomers apart: each gives a different free ion in water.
Definition
- Linkage: an ambidentate ligand bonds through a different atom: (nitro, N-bonded, yellow) and (nitrito, O-bonded, red).
- Ionisation: a ligand inside and an ion outside the bracket exchange: (gives AgBr with ) and (gives with ).
- Coordination (NCERT definition): ligands exchange between the cationic and the anionic complex of DIFFERENT metal ions: and .
- Solvate (hydrate): water inside versus outside the sphere: (violet), , .
| Type | What changes | Example pair or member | How to tell |
|---|---|---|---|
| Linkage | Donor atom of an ambidentate ligand | and | Colour and infrared spectrum differ |
| Ionisation | Which anion is inside the bracket | and | test for the halide, test for sulphate |
| Coordination | Which metal holds which ligands | and | Needs a complex cation AND a complex anion of different metals |
| Solvate (hydrate) | Water inside or outside the sphere | and | Number of chlorides precipitated by (3 and 2) |
Ionisation and hydrate isomers are told apart by what precipitates; linkage isomers need an ambidentate ligand.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2025 · 7 Apr 2025 · Q44Moderate
Example 1 · Coordination Compounds · Isomerism
An octahedral complex having molecular composition has two isomers A and B . The solution of A gives a white precipitate with solution and the solution of B gives white precipitate with solution. The type of isomerism exhibited by the complex is,
Coordination isomerism needs two different metals
NCERT defines coordination isomerism as an exchange of ligands between the cationic and anionic complexes of DIFFERENT metal ions. By that definition , with cobalt in both ions, is not counted. A 2026 key follows this definition.
Ionisation isomers give different ions, not different amounts
gives chloride and gives sulphate. If one isomer precipitates with and the other with , the isomerism is ionisation, not linkage or coordination.
Concept 2 of 3: Geometrical isomers of square planar and octahedral complexes
Geometrical isomers differ in which ligands are next to each other (cis, 90° apart) and which are opposite (trans, 180°). A tetrahedron has every corner next to every other, so it never has cis–trans isomers. In an octahedron, three identical ligands can sit on one face (fac) or along a meridian through the metal (mer).
Definition
- Tetrahedral (sp³) complexes: no geometrical isomers, whatever the ligands.
- Square planar and : cis and trans, 2 isomers. : 3 isomers (fix A; B, C or D can be trans to it).
- Octahedral and : cis and trans, 2 isomers.
- Octahedral : fac (all B–M–B angles 90°) and mer (B–M–B angles 90° and 180°), 2 isomers.
- , and : no geometrical isomers.
| Type | Example | Geometrical isomers | Names |
|---|---|---|---|
| Tetrahedral | An sp³ complex MABXL | 0 | All corners equivalent |
| Square planar | 2 | cis and trans (cisplatin is the cis form) | |
| Square planar | 3 | Each of B, C, D trans to A in turn | |
| Octahedral | 2 | cis and trans | |
| Octahedral | 2 | fac and mer | |
| Octahedral | 2 | cis and trans (cis is also chiral) | |
| Octahedral | 0 | Only optical isomers | |
| Octahedral | 0 | One odd ligand has only one kind of position |
Count geometrical isomers from the formula type; the metal and the ligands' names do not matter.
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The same idea in a real exam question:
JEE Mains · 2023 · 11 April 2023 · Q45Moderate
Example 2 · Coordination Compounds · Isomerism
Which of the following complex has a possibility to exist as meridional isomer?
Tetrahedral complexes have no cis–trans isomers
Every pair of corners of a tetrahedron is adjacent, so no ligand can be trans to another. An sp³ complex MABXL has 0 geometrical isomers, even with four different ligands.
Do not count the cis enantiomers as geometrical isomers
has 2 geometrical isomers, cis and trans. The cis form also has a mirror image, which makes 3 stereoisomers, but the extra one is an optical isomer.
Changing Ni²⁺ to Pt²⁺ changes the geometry
is tetrahedral and paramagnetic with no geometrical isomers. is square planar, diamagnetic and has cis and trans forms.
Concept 3 of 3: Optical isomers and total stereoisomers of octahedral complexes
A complex is optically active when it has no mirror plane and no centre of symmetry, so its mirror image cannot be laid on it. Chelate rings wrapped around an octahedron like a propeller do this: the cis forms of and every come as left- and right-handed pairs. To count stereoisomers, list the geometrical isomers first, then double each one that is chiral.
Definition
- , such as or : one geometrical form, chiral, so 2 stereoisomers (d and l).
- and : trans is achiral (a mirror plane holds both rings); cis is chiral. Total 3 stereoisomers.
- : fac and mer are both achiral, so 2 stereoisomers.
- Square planar complexes are flat and always have a mirror plane: never optically active.
Counting stereoisomers
Worked example
How many stereoisomers does have, and how many of them are optically active?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
JEE Mains · 2023 · 13 April 2023 · Q125Moderate
Example 3 · Coordination Compounds · Isomerism
The total number of stereoisomers for the complex where ox oxalate is:
A stereoisomer count includes the optical isomers
has 2 geometrical isomers but 3 stereoisomers, because its cis form is chiral. has 2 stereoisomers, since fac and mer are both achiral.
Look for the mirror plane, not for four different groups
The carbon rule of four different groups does not apply to complexes. cis-, with only two kinds of ligand, is chiral; trans- is not.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Optical isomers and total stereoisomers of octahedral complexes
Counting stereoisomers
Reference tables (2)
Types of structural isomerism in complexes4 rows
| Type | What changes | Example pair or member | How to tell |
|---|---|---|---|
| Linkage | Donor atom of an ambidentate ligand | and | Colour and infrared spectrum differ |
| Ionisation | Which anion is inside the bracket | and | test for the halide, test for sulphate |
| Coordination | Which metal holds which ligands | and | Needs a complex cation AND a complex anion of different metals |
| Solvate (hydrate) | Water inside or outside the sphere | and | Number of chlorides precipitated by (3 and 2) |
Ionisation and hydrate isomers are told apart by what precipitates; linkage isomers need an ambidentate ligand.
Geometrical isomers of square planar and octahedral complexes8 rows
| Type | Example | Geometrical isomers | Names |
|---|---|---|---|
| Tetrahedral | An sp³ complex MABXL | 0 | All corners equivalent |
| Square planar | 2 | cis and trans (cisplatin is the cis form) | |
| Square planar | 3 | Each of B, C, D trans to A in turn | |
| Octahedral | 2 | cis and trans | |
| Octahedral | 2 | fac and mer | |
| Octahedral | 2 | cis and trans (cis is also chiral) | |
| Octahedral | 0 | Only optical isomers | |
| Octahedral | 0 | One odd ligand has only one kind of position |
Count geometrical isomers from the formula type; the metal and the ligands' names do not matter.
Watch out for (7)
- Coordination isomerism needs two different metals→ Types of structural isomerism in complexes
- Ionisation isomers give different ions, not different amounts→ Types of structural isomerism in complexes
- Tetrahedral complexes have no cis–trans isomers→ Geometrical isomers of square planar and octahedral complexes
- Do not count the cis enantiomers as geometrical isomers→ Geometrical isomers of square planar and octahedral complexes
- Changing Ni²⁺ to Pt²⁺ changes the geometry→ Geometrical isomers of square planar and octahedral complexes
- A stereoisomer count includes the optical isomers→ Optical isomers and total stereoisomers of octahedral complexes
- Look for the mirror plane, not for four different groups→ Optical isomers and total stereoisomers of octahedral complexes
Test yourself on Coordination Compounds
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