Isomerism in Coordination Compounds
The ultimate JEE reference for stereochemistry. Master the geometrical (cis/trans, fac/mer), optical (d/l pairs), and structural isomerism of the 50 most crucial complexes.
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1. Square Planar (C.N. = 4)
$$\ce{[Pt(NH3)2Cl2]}$$
Type: $MA_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0 (Planar symmetry)
STR Notes
Cis isomer is Cisplatin
Total Stereoisomers = 2
$$\ce{[Pt(NH3)(py)ClBr]}$$
Type: $MABCD$
GEO Geometrical
3 Isomers
OPT Optical
0 (Planar symmetry)
STR Notes
Fix one ligand, rotate others
Total Stereoisomers = 3
$$\ce{[Pt(gly)2]}$$
Type: $M(AB)_2$ (Unsymmetrical)
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0 (Planar)
STR Notes
Trans has center of inversion
Total Stereoisomers = 2
$$\ce{[Pd(NH3)2(NO2)2]}$$
Type: $MA_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0
STR Structural
Linkage ($NO_2$ vs $ONO$)
Total Stereoisomers = 2
$$\ce{[Pt(en)Cl2]}$$
Type: $M(AA)B_2$
GEO Geometrical
0 (No Trans possible)
OPT Optical
0
STR Notes
en ligand cannot span 180°
Total Stereoisomers = 1
$$\ce{[Zn(NH3)2Cl2]}$$
Type: Tetrahedral $MA_2B_2$
GEO Geometrical
0
OPT Optical
0
STR Notes
Tetrahedral geometry prevents GI
Total Stereoisomers = 1
$$\ce{[Pd(C6H5)2(dppe)]}$$
Type: $M(AA)B_2$
GEO Geometrical
0
OPT Optical
0
STR Notes
dppe forces cis geometry only
Total Stereoisomers = 1
$$\ce{[Pt(py)4][PtCl4]}$$
Type: Bimetallic
GEO Geometrical
0
OPT Optical
0
STR Structural
Coordination Isomerism
Forms $[Pt(py)_3Cl][Pt(py)Cl_3]$
$$\ce{[Pd(bipy)(SCN)2]}$$
Type: $M(AA)B_2$
GEO Geometrical
0
OPT Optical
0
STR Structural
Linkage Isomerism (SCN/NCS)
Total Stereoisomers = 1
$$\ce{[Ni(dmg)2]}$$
Type: $M(AB)_2$ (H-Bonded)
GEO Geometrical
0 (Practically)
OPT Optical
0
STR Notes
Trans structure locked by H-bonds
Total Stereoisomers = 1
2. Octahedral Monodentate
$$\ce{[Co(NH3)4Cl2]^+ }$$
Type: $MA_4B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0 (Both Achiral)
STR Colors
Cis: Violet, Trans: Green
Total Stereoisomers = 2
$$\ce{[Co(NH3)3(NO2)3]}$$
Type: $MA_3B_3$
GEO Geometrical
2 (Facial, Meridional)
OPT Optical
0 (Both Achiral)
STR Structural
Linkage ($NO_2$ vs $ONO$)
Total Stereoisomers = 2
$$\ce{[Pt(NH3)2Cl2Br2]}$$
Type: $MA_2B_2C_2$
GEO Geometrical
5 Isomers
OPT Optical
1 Pair (Cis-Cis-Cis is Chiral)
STR Notes
All Trans, 2 Trans, All Cis
Total Stereoisomers = 6
$$\ce{[Cr(NH3)4ClBr]^+ }$$
Type: $MA_4BC$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0 (Both Achiral)
STR Notes
Cl and Br position defines isomer
Total Stereoisomers = 2
$$\ce{[Co(NH3)5Cl]^2+}$$
Type: $MA_5B$
GEO Geometrical
0
OPT Optical
0
STR Notes
Requires at least 2 differing ligands
Total Stereoisomers = 1
$$\ce{[Rh(py)3Cl3]}$$
Type: $MA_3B_3$
GEO Geometrical
2 (fac, mer)
OPT Optical
0
STR Notes
Fac = corners of one triangular face
Total Stereoisomers = 2
$$\ce{[Pt(NH3)2(py)2Cl2]^2+}$$
Type: $MA_2B_2C_2$
GEO Geometrical
5 Isomers
OPT Optical
1 Pair (All-Cis is Chiral)
STR Notes
Common high-level JEE question
Total Stereoisomers = 6
$$\ce{[Pt(NH3)(py)(NH2OH)(NO2)ClBr]}$$
Type: $MABCDEF$
GEO Geometrical
15 Isomers
OPT Optical
15 Pairs (All Chiral)
STR Notes
Maximum possible stereoisomers
Total Stereoisomers = 30
$$\ce{[Co(NH3)4(NO2)2]^+ }$$
Type: $MA_4B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
0
STR Structural
Linkage Isomerism possible
Total Stereoisomers = 2
$$\ce{[Cr(NH3)5(SCN)]^2+}$$
Type: $MA_5B$
GEO Geometrical
0
OPT Optical
0
STR Structural
Linkage (Thiocyanato/Isothiocyanato)
Total Stereoisomers = 1
3. Octahedral Chelating (Optical Focus)
$$\ce{[Co(en)3]^3+}$$
Type: $M(AA)_3$
GEO Geometrical
0
OPT Optical
1 Pair (d, l)
STR Notes
Propeller-like chirality
Total Stereoisomers = 2
$$\ce{[Co(en)2Cl2]^+ }$$
Type: $M(AA)_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
Cis is Chiral (d, l pair)
STR Notes
Trans is Achiral (plane of sym)
Total Stereoisomers = 3
$$\ce{[Cr(ox)2(H2O)2]^-}$$
Type: $M(AA)_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
Cis is Chiral (d, l pair)
STR Notes
Classic JEE example
Total Stereoisomers = 3
$$\ce{[Co(en)2ClBr]^+ }$$
Type: $M(AA)_2BC$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
Cis is Chiral (d, l pair)
STR Notes
Trans is Achiral
Total Stereoisomers = 3
$$\ce{[Co(gly)3]}$$
Type: $M(AB)_3$ (Unsymmetrical)
GEO Geometrical
2 (Facial, Meridional)
OPT Optical
Both Fac & Mer are Chiral!
STR Notes
Highly tested exception rule
Total Stereoisomers = 4
$$\ce{[Pt(en)(NH3)2Cl2]^2+}$$
Type: $M(AA)B_2C_2$
GEO Geometrical
3 Isomers
OPT Optical
1 Pair (Cis-Cis is Chiral)
STR Notes
Trans-$NH_3$ and Trans-Cl are achiral
Total Stereoisomers = 4
$$\ce{[Co(EDTA)]^-}$$
Type: Hexadentate
GEO Geometrical
0
OPT Optical
1 Pair (d, l)
STR Notes
EDTA wraps in a chiral conformation
Total Stereoisomers = 2
$$\ce{[Co(dien)2]^3+}$$
Type: $M(ABA)_2$ (Tridentate)
GEO Geometrical
3 (s-fac, u-fac, mer)
OPT Optical
u-fac and mer are Chiral!
STR Notes
s-fac is Achiral. Advanced topic.
Total Stereoisomers = 5
$$\ce{[Cr(ox)3]^3-}$$
Type: $M(AA)_3$
GEO Geometrical
0
OPT Optical
1 Pair (d, l)
STR Notes
Highly stable optically active salt
Total Stereoisomers = 2
$$\ce{[Ru(bipy)3]^2+}$$
Type: $M(AA)_3$
GEO Geometrical
0
OPT Optical
1 Pair ($\Delta, \Lambda$ notation)
STR Notes
Famous luminescent complex
Total Stereoisomers = 2
4. Structural Isomerism Focus
$$\ce{[Co(NH3)5Br]SO4}$$
Type: Ionization
STR Isomer
$\ce{[Co(NH3)5SO4]Br}$
GEO Geometrical
0
STR Test
$BaCl_2$ (ppt) vs $AgNO_3$ (ppt)
Ionization Isomers
$$\ce{[Cr(H2O)6]Cl3}$$
Type: Solvate / Hydration
STR Isomer 1
$\ce{[Cr(H2O)5Cl]Cl2.H2O}$
STR Isomer 2
$\ce{[Cr(H2O)4Cl2]Cl.2H2O}$
STR Colors
Violet $\rightarrow$ Blue-Green $\rightarrow$ Dark Green
Hydration Isomers
$$\ce{[Co(NH3)6][Cr(CN)6]}$$
Type: Coordination
STR Isomer 1
$\ce{[Cr(NH3)6][Co(CN)6]}$
STR Isomer 2
$\ce{[Co(NH3)5CN][Cr(NH3)(CN)5]}$
STR Notes
Interchange of ligands between metals
Coordination Isomers
$$\ce{[Co(en)2(NO2)Cl]SCN}$$
Type: Mixed Structural
STR Ionization
$\ce{[Co(en)2(NO2)SCN]Cl}$
STR Linkage
Use of $ONO$ or $NCS$
GEO Geo/Opt
Complex exhibits ALL types
Ultimate Isomerism Example
$$\ce{[Pt(NH3)4Cl2]Br2}$$
Type: Ionization
STR Isomer
$\ce{[Pt(NH3)4Br2]Cl2}$
GEO Geometrical
Both have Cis/Trans forms
STR Test
$AgNO_3$ yields AgBr vs AgCl
Ionization + Geometrical
$$\ce{[Co(NH3)5(NO2)]Cl2}$$
Type: Linkage
STR Isomer
$\ce{[Co(NH3)5(ONO)]Cl2}$
STR Colors
Nitro (Yellow) vs Nitrito (Red)
GEO Geometrical
0
Linkage Isomers
$$\ce{[Cu(NH3)4][PtCl4]}$$
Type: Coordination
STR Isomer
$\ce{[Pt(NH3)4][CuCl4]}$
GEO Geometrical
0 (Square planar complexes)
STR Notes
Metals trade all ligands
Coordination Isomers
$$\ce{[Pt(NH3)4][PtCl6]}$$
Type: Coordination Position
STR Isomer
$\ce{[Pt(NH3)4Cl2][PtCl4]}$
STR States
Pt(II)/Pt(IV) vs Pt(IV)/Pt(II)
GEO Geometrical
Yes, in the $MA_4B_2$ part
Coordination Isomers
$$\ce{[Co(NH3)5(SCN)]Cl2}$$
Type: Linkage
STR Isomer
$\ce{[Co(NH3)5(NCS)]Cl2}$
STR Ligand
Thiocyanato vs Isothiocyanato
GEO Geometrical
0
Linkage Isomers
$$\ce{[Cr(H2O)5(SCN)]Cl2}$$
Type: Mixed Structural
STR Ionization
$\ce{[Cr(H2O)5Cl]Cl(SCN)}$
STR Linkage
Uses NCS instead of SCN
STR Hydration
$\ce{[Cr(H2O)4Cl(SCN)]Cl.H2O}$
Multiple Structural Isomers
5. Special & Tricky Cases
$$\ce{[Ni(CO)4]}$$
Type: Tetrahedral
GEO Geometrical
0
OPT Optical
0
STR Notes
Perfect tetrahedron prevents isomerism
Total Stereoisomers = 1
$$\ce{[Fe(CN)5(NO)]^2-}$$
Type: $MA_5B$ (Nitroprusside)
GEO Geometrical
0
OPT Optical
0
STR Notes
Highly symmetric except for NO axis
Total Stereoisomers = 1
$$\ce{[Cr(en)2(SCN)2]^+ }$$
Type: $M(AA)_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
Cis is Chiral
STR Structural
Linkage Isomerism also possible
Total Stereoisomers = 3
$$\ce{[Fe(acac)3]}$$
Type: $M(AA)_3$
GEO Geometrical
0
OPT Optical
1 Pair (d, l)
STR Notes
acac is a symmetrical bidentate
Total Stereoisomers = 2
$$\ce{[Co(NH3)5(H2O)]Cl3}$$
Type: $MA_5B$
GEO Geometrical
0
OPT Optical
0
STR Notes
Rose-pink colored complex
Total Stereoisomers = 1
$$\ce{[Pt(NH3)3Cl]^+ }$$
Type: Square Planar $MA_3B$
GEO Geometrical
0
OPT Optical
0
STR Notes
Requires at least $MA_2B_2$ for GI
Total Stereoisomers = 1
$$\ce{[Co(en)3]Cl3}$$
Type: $M(AA)_3$
GEO Geometrical
0
OPT Optical
1 Pair (d, l)
STR Notes
Can be resolved via fractional crystallization
Total Stereoisomers = 2
$$\ce{[Pt(pn)2]^2+}$$
Type: Square Planar Chelated
GEO Geometrical
0
OPT Optical
Yes (Ligand is Chiral)
STR Notes
Propylenediamine itself is optically active
Depends on ligand stereocenters
$$\ce{[Rh(en)2Cl2]^+ }$$
Type: $M(AA)_2B_2$
GEO Geometrical
2 (Cis, Trans)
OPT Optical
Cis is Chiral
STR Notes
Identical stereochemistry to Cobalt analog
Total Stereoisomers = 3
$$\ce{[Ir(H2O)3(NO2)3]}$$
Type: $MA_3B_3$
GEO Geometrical
2 (fac, mer)
OPT Optical
0
STR Notes
Meridional = ligands form a T-shape/Meridian
Total Stereoisomers = 2
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