How to find number of Geometrical Isomers | Stereoisomerism | Coordination Compounds IIT JEE NEET 😊👍

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Geometrical Isomers in Coordination Chemistry

1. What are Geometrical Isomers?
Geometrical isomers are compounds that have the same chemical formula and the same bonds between atoms, but the arrangement of ligands around the central metal atom/ion is different in space.
Because of this difference in arrangement, their physical and chemical properties can be different, even though they contain the same atoms.
This is a type of stereoisomerism — isomers differ only in the spatial arrangement of ligands, not in connectivity.

2. Why do they occur?
Geometrical isomers occur when:
The ligands are fixed in space by the coordination geometry (shape) of the complex.
There is restricted rotation around metal–ligand bonds (unlike simple organic molecules where bonds can rotate easily).
Different positions of ligands create distinct, stable arrangements.
3. Common Geometrical Isomers
(a) Cis–Trans Isomerism
Found mainly in square planar and octahedral complexes.
Cis = two same ligands are next to each other (90° apart).
Trans = two same ligands are opposite each other (180° apart).
Examples:
Square planar:
[Pt(NH3)2Cl2][Pt(NH3)2Cl2]
Cis: both Cl atoms next to each other.
Trans: Cl atoms opposite each other.
Octahedral:
[Co(NH3)4Cl2]+[Co(NH3)4Cl2] +

Cis: two Cl atoms next to each other.
Trans: two Cl atoms opposite each other.
(b) Facial–Meridional (fac–mer) Isomerism
Occurs in octahedral complexes of the type
MA3B3MA3B3
​
(three ligands of type A, three of type B).
Facial (fac): three identical ligands occupy one face of the octahedron — they are all 90° from each other.
Meridional (mer): three identical ligands lie along a meridian — two are 90° apart, and the third is 180° from one of them.
Example:
[Co(NH3)3(NO2)3][Co(NH3)3(NO2)3]
fac-isomer: three NO₂ groups form a triangular face.
mer-isomer: three NO₂ groups in an L-shaped arrangement with one opposite another.
4. Geometrical Isomerism in Different Geometries
(i) Octahedral
Cis–trans isomers possible for complexes of type
MA4B2MA4B2

Fac–mer isomers possible for
MA3B3MA3B3

More complex patterns possible with mixed ligands (e.g.,
MA2B2C2MA2B2C2).
(ii) Square Planar
Only cis–trans isomerism possible.
Common for MA2B2MA2B2 complexes.
No fac–mer here because "faces" don’t exist in 2D geometry.
(iii) Tetrahedral
Geometrical isomerism is rare because all bond angles are the same (109.5°).
Usually, no cis–trans distinction is possible for simple tetrahedral complexes — only optical isomers are possible when ligands differ.
5. Why is this important?
Geometrical isomers can have different reactivity, color, solubility, and biological activity.
Example: cisplatin
[Pt(NH3)2Cl2][Pt(NH3)2Cl 2] is a powerful anti-cancer drug, while transplatin is not effective and more toxic.

Cis–trans = position of same ligands: next to or opposite each other.
Fac–mer = arrangement of three identical ligands in an octahedron: all on one face or spread along a meridian.
Shape of the complex (octahedral, square planar, tetrahedral) determines which types of geometrical isomers are possible.

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