Isomerism in Coordination Compounds
Decode spatial arrangements and chemical structures. Master structural isomers, fac/mer geometries, and the definitive rules for optical activity in octahedral complexes.
Module Focus: Same Formula, Different Architecture
Isomers are compounds with the exact same molecular formula but different arrangements of atoms. In coordination chemistry, this is split into two major branches: Structural Isomerism (differences in which bonds are formed) and Stereoisomerism (differences in 3D spatial arrangement without changing connectivity). NEET heavily tests your ability to identify these isomers based on chemical precipitation tests and symmetry planes.
1. Structural Isomerism
Structural isomers have different bonding connectivity. There are four primary types you must be able to identify instantly.
Exchange of ions inside the coordination sphere with counter ions outside the sphere.
(Gives white ppt with $BaCl_2$)
$[Co(NH_3)_5SO_4]Br$
(Gives pale yellow ppt with $AgNO_3$)
Arises exclusively when an ambidentate ligand is present, capable of coordinating through two different donor atoms.
(Binds via Nitrogen: Yellow)
$[Co(NH_3)_5(\mathbf{ONO})]Cl_2$
(Binds via Oxygen: Red)
Occurs when BOTH cation and anion are complex ions. Ligands are interchanged between the two metal centers.
vs
$[Cr(NH_3)_6] [Co(CN)_6]$
A specific type of ionization isomerism where water ($H_2O$) molecules exchange places between inside and outside the sphere.
$[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$ (Grey-green)
$[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O$ (Dark green)
2. Geometrical Isomerism (cis/trans)
Occurs due to different spatial arrangements of ligands around the central metal atom. Ligands can be adjacent (cis) or opposite (trans) to each other.
Tetrahedral complexes (C.N. = 4, $sp^3$) DO NOT show geometrical isomerism because all four positions are adjacent to one another and equidistant. Relative positions cannot be changed.
A. Square Planar Complexes (C.N. = 4)
Shows cis and trans isomerism. The classic example is the anti-cancer drug Cisplatin.
- $Ma_4$ (e.g., $[PtCl_4]^{2-}$)
- $Ma_3b$ (e.g., $[Pt(NH_3)_3Cl]^+$)
Because swapping any two ligands results in the exact same spatial arrangement.
B. Octahedral Complexes (C.N. = 6)
e.g., $[Co(NH_3)_4Cl_2]^+$
- Cis: The two $Cl^-$ are at $90^\circ$ to each other.
- Trans: The two $Cl^-$ are opposite at $180^\circ$.
e.g., $[Co(NH_3)_3Cl_3]$
- Facial (fac): 3 identical ligands occupy corners of the same triangular face of the octahedron.
- Meridional (mer): 3 identical ligands lie around the meridian (equator) of the octahedron.
3. Optical Isomerism (Chirality)
Optical isomers (enantiomers) are non-superimposable mirror images. They rotate plane-polarized light in opposite directions (dextro 'd' and laevo 'l'). To show optical isomerism, the complex must lack a plane of symmetry.
Square Planar Complexes: Almost never show optical isomerism because all ligands and the metal lie in the exact same plane, which acts as a plane of symmetry.
Optical isomerism is extremely common in octahedral complexes containing di- or polydentate ligands (like ethylenediamine, 'en', or oxalate, 'ox').
e.g., $[Co(en)_3]^{3+}$
Always Optically Active.
Forms non-superimposable 'd' and 'l' mirror images.
e.g., $[Co(en)_2Cl_2]^+$
- Cis-isomer is Optically ACTIVE (resolves into d & l).
- Trans-isomer is Optically INACTIVE (has a plane of symmetry).
(1 Trans + 2 Cis enantiomers)
NEET Grand Test: Isomerism
15 High-Yield Questions testing structural identification, fac/mer geometries, and optical symmetry planes.
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