2pt)26ptIsomerism in Coordination Compounds
JEE Mains/Advanced Ready Notes — High-Yield Concepts, Rules, and Examples
Core Idea
Isomerism in coordination compounds refers to compounds having the same molecular formula but dif-
ferent arrangements of atoms/ligands, leading to distinct properties. It splits into:
• Structural (Constitutional): different connectivity.
• Stereoisomerism: same connectivity but different spatial arrangement.
Classification Overview
• Structural – (Rare) Polymerization isomerism
– Ionization isomerism
• Stereoisomerism
– Hydrate (solvate) isomerism
– Coordination isomerism – Geometrical (cis/trans, fac/mer, E/Z)
– Linkage isomerism – Optical (enantiomers, Λ/∆)
Structural Isomerism
Ionization Isomerism
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Definition Differences arise from inter-exchange of counter ion and ligand within the coordination sphere.
They yield different ions in solution.
Example:
[Co(NH3 )5 SO4 ]Br vs · [Co(NH3 )5 Br]SO4
Tests: AgNO3 gives AgBr with first; BaCl2 gives BaSO4 with second.
Hydrate (Solvate) Isomerism
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Definition Interchange between coordinated water and water of crystallization.
Example (classic):
[Cr(H2 O)6 ]Cl3 , [Cr(H2 O)5 Cl]Cl2 · H2 O, [Cr(H2 O)4 Cl2 ]Cl · 2 H2 O
Different number of ions in solution and differing colors/conductance.
Coordination Isomerism
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Definition Occurs in salts with complex cation and complex anion; ligands redistribute between metal
centers.
[Co(NH3 )6 ][Cr(CN)6 ] ↔ [Cr(NH3 )6 ][Co(CN)6 ]
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Linkage Isomerism
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Definition Ambidentate ligands can bind through two different donor atoms.
Key ligands:
• NO2 – : nitro ( – NO2 – N) vs nitrito-κO ( – ONO)
• SCN – : thiocyanato-κS ( – S – C) vs isothiocyanato-κN ( – N – C – S)
• CN – /NC – (rare “cyanide/isocyanide” in complexes)
Example:
[Co(NH3 )5 (NO2 )]2+ (nitro) vs · [Co(NH3 )5 (ONO)]2+ (nitrito)
Stereoisomerism — Geometrical
Square Planar (d8 like Pt2+ , Pd2+ )
• MA2 B2 : cis/trans
• MABCD: up to 3 geometrical isomers (no optical activity in ideal D4h duetomirrorplane).
Quick Test
In square planar MA2 B2 : cis isomers have identical ligands adjacent; trans have them opposite. cis often
more polar and reactive (e.g., ).
Octahedral (Most Common)
• MA4 B2 : cis/trans
• MA3 B3 : fac/mer
• M(AA)2 B2 : multiple cis/transarrangements; possible optical activity for certain combinations.
• M(AA)3 : typically optically active (Λ/∆).
fac vs mer
• fac: three identical ligands at the corners of one octahedral face (90° between each pair).
• mer: three identical ligands in a meridian (two trans, one cis).
Useful TikZ Sketches
OctahedralBframework
A
A B
A
Example: cis-MA4 B2
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Stereoisomerism — Optical
Definition
Optical isomers (enantiomers) are non-superimposable mirror images that rotate plane-polarized light
by equal magnitude in opposite directions. They have identical physical properties except optical rota-
tion and behavior in chiral environments.
When do octahedral complexes show optical activity?
• M(AA)3 (three bidentates like en, bpy): Λ and ∆ pair.
• MA2 (AA)2 (cis) often optically active; trans usually not (plane/center of symmetry present).
• MABC(AA) (cis) can be chiral.
• MA2 B2 (AA) (cis) sometimes chiral; trans generally achiral.
• MA3 B3 : fac often chiral if all three A are chelating and oriented to remove improper axes; simple
monodentate fac/mer are achiral.
Λ/∆ Convention
For tris-chelates M(AA)3 , the helical twist of chelate rings around the metal defines configuration:
• Λ: left-handed helix.
• ∆: right-handed helix.
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High-Yield Examples
• [M(en)3 ]n+ : Λ and ∆ enantiomers; optically active.
• [Cr(ox)3 ]3 – (ox = C2 O4 2 – ): optically active.
• [Pt(en)2 Cl2 ]2+ : cis is optically active; trans is achiral.
• [M(AA)2 AB]n+ : often chiral if cis.
Detection and Distinction
Geometrical
• Polarity/Conductance: cis often more polar than trans.
• Chemical Reactivity: [Pt(NH3 )2 Cl2 ] (cis) is chemotherapeutic; trans is inactive.
• Spectroscopy: Different splitting/patterns in NMR (for diamagnetic complexes).
Optical
• Polarimetry: non-zero specific rotation.
• Resolution: via chiral counter-ions or chiral chromatography.
Decision Flow (Exam Shortcut)
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Is it geometrical? Square planar: MA2 B2 or MABCD ⇒ likely cis/trans.
Octahedral: MA4 B2 (cis/trans), MA3 B3 (fac/mer).
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Is it optical? Octahedral with chelates: M(AA)3 always; cis M(AA)2 B2 often; trans usually not. Presence
of plane/center of symmetry kills optical activity.
Worked Examples (JEE Style)
1) Count geometrical isomers for [MA4 B2 ] (octahedral)
Answer: 2 (cis, trans).
Reason: Two B ligands can be adjacent (90°) or opposite (180°).
2) [M(A)3 (B)3 ]: identify fac/mer
Answer: Two isomers: fac (all B on one face) and mer (B in a meridian).
Tip: fac has 3 B mutually 90°; mer has one B trans to another.
3) Which is optically active: [M(en)2 Cl2 ]2+ cis or trans?
Answer: cis is optically active; trans is not.
Reason: trans has mirror plane/center, removing chirality.
4) Identify linkage isomer pairs
• [Co(NH3 )5 (NO2 )]2+ vs [Co(NH3 )5 (ONO)]2+ : nitro/nitrito.
• [Pt(NH3 )4 (SCN)]2+ vs [Pt(NH3 )4 (NCS)]2+ : thiocyanato/isothiocyanato.
5) Ionization vs hydrate
• Ionization: [Co(NH3 )5 SO4 ]Br releases Br – on dissolution.
• Hydrate: [Cr(H2 O)5 Cl]Cl2 · H2 O differs by coordinated vs lattice water.
Common Pitfalls and Tips
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Avoid These Errors
• Assuming tetrahedral complexes show cis/trans: they do not for MA2 B2 (tetrahedral lacks such
distinction).
• Assuming all octahedral MA2 B2 are optically active: only some cis forms with chelates;
monodentate-only cis MA2 B2 is usually achiral.
• Confusing fac/mer with cis/trans: fac/mer applies to MA3 B3 (octahedral).
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Memory Hooks
• Square planar → think : cis active, trans inactive.
• fac = face (all three on one face); mer = meridian (line through center).
• Linkage = ligand donor switch (N/O in NO2 – , S/N in SCN – ).
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Quick Table
Type Key Pattern Hallmarks
Ionization Counter ion swap Different ions in solution (Ag+ /Ba2+
tests)
Hydrate Coord vs lattice H2 O Different conductivity, color, water loss
Coordination Cation/anion both complex Ligand redistribution between metals
Linkage Ambidentate donors Nitro/nitrito; SCN/NCS pairs
Geometrical cis/trans, fac/mer Square planar MA2 B2 ; Octahedral
MA4 B2 , MA3 B3
Optical Chiral arrangements No plane/center; Λ/∆ for tris-chelates
Practice Set (Try Quickly)
1. Number of isomers (geometrical + optical) for [M(en)2 Cl2 ]2+ ?
2. Which shows linkage isomerism: [Co(NH3 )5 Cl]Cl2 , [Co(NH3 )5 (NO2 )]Cl2 , [Cr(H2 O)6 ]Cl3 ?
3. Distinguish [Co(NH3 )5 SO4 ]Br and [Co(NH3 )5 Br]SO4 experimentally.
4. Predict whether [M(AA)3 ] is optically active and assign Λ/∆.
5. For [MA3 B3 ], sketch fac and mer in octahedral geometry.
Prepared as concise JEE-focused notes. Compile with XeLaTeX for best results.
