Question
Explain geometrical and optical isomerism in coordination compounds with examples. Can show both types of isomerism?
(NEET 2022, similar pattern)
Solution — Step by Step
Geometrical isomerism arises when ligands can be arranged differently in space around the central metal ion, leading to cis (same side) and trans (opposite side) forms.
Condition: The complex must have at least two different types of ligands in a square planar or octahedral geometry.
Example: (square planar)
- cis isomer: both Cl atoms on the same side
- trans isomer: Cl atoms on opposite sides
Note: Tetrahedral complexes do NOT show geometrical isomerism (all positions are equivalent).
Optical isomerism occurs when a complex and its mirror image are non-superimposable. Such pairs are called enantiomers. One rotates plane-polarised light clockwise (dextro, d) and the other anticlockwise (laevo, l).
Condition: The complex must lack a plane of symmetry and a centre of symmetry (it must be chiral).
This is most commonly seen in octahedral complexes with bidentate ligands like ethylenediamine (en).
This is an octahedral complex with 2 bidentate en ligands and 2 monodentate Cl⁻ ligands.
Geometrical isomerism: Yes.
- cis isomer: both Cl⁻ on the same side (adjacent positions)
- trans isomer: both Cl⁻ on opposite sides (180° apart)
Optical isomerism: Only the cis isomer shows optical isomerism (it has no plane of symmetry). The trans isomer has a plane of symmetry and is optically inactive.
So has three isomers in total: cis-d, cis-l, and trans.
Why This Works
Geometrical isomerism is about spatial arrangement — same connectivity but different 3D positioning. Optical isomerism is about chirality — the molecule and its mirror image cannot be superimposed.
The trans isomer of has a mirror plane passing through the two Cl atoms and the Co centre, making it achiral. The cis isomer lacks such a plane — the en ligands wrap around the metal in a way that creates a chiral centre, giving rise to d and l forms.
Alternative Method — Quick Rules
| Geometry | Geometrical Isomerism | Optical Isomerism |
|---|---|---|
| Tetrahedral | No | Possible (rare) |
| Square planar | Yes (for ) | No (planar = has symmetry plane) |
| Octahedral | Yes | Yes (especially with bidentate ligands) |
For NEET, remember: shows only optical isomerism (no geometrical — all ligands are identical bidentate). shows both geometrical and optical isomerism. These two are the most frequently tested examples.
Common Mistake
Students often claim that the trans isomer of also shows optical isomerism. It does NOT. The trans isomer has a plane of symmetry and is superimposable on its mirror image. Only the cis isomer is chiral. Always check for a symmetry plane before claiming optical activity.