Optical Isomerism
Stereoisomerism in Coordination Compounds | Class 12
1. Conditions for Optical Activity
The primary condition for optical activity is the Absence of a Plane of Symmetry (POS).
- If a molecule has a POS, it is Achiral (Optically Inactive).
- If a molecule lacks a POS, it is Chiral (Optically Active).
Optical isomers exist as pairs of Enantiomers:
1. Dextro (d): Rotates plane-polarized light to the right.
2. Levo (l): Rotates plane-polarized light to the left.
2. Coordination Number 4
A. Square Planar Complexes
Generally Optically Inactive.
B. Tetrahedral Complexes
Can show optical isomerism if all four ligands are different ($Mabcd$), analogous to a chiral carbon atom. (e.g., Bis(benzoylacetonato)beryllium(II)).
3. Coordination Number 6 (Octahedral)
Optical isomerism is very common here, especially with bidentate ligands (like $en$, $ox$).
Type A: $[M(AA)_3]$
Example: $[Co(en)_3]^{3+}$ or $[Cr(ox)_3]^{3-}$.
- Contains 3 symmetrical bidentate ligands.
- Optically Active: It has a "Propeller" shape and lacks a plane of symmetry.
- Exists as 'd' and 'l' forms.
Type B: $[M(AA)_2B_2]$
Example: $[Co(en)_2Cl_2]^+$
| Isomer | Structure | Optical Nature |
|---|---|---|
| Cis | Chlorines are adjacent ($90^\circ$). | Optically Active (No POS). Exists as d/l pair. |
| Trans | Chlorines are opposite ($180^\circ$). | Optically Inactive (Meso-like). Has a Plane of Symmetry. |
Type C: $[M(AA)B_2C_2]$
Example: $[Co(en)(NH_3)_2Cl_2]^+$
Shows both Geometrical and Optical isomerism. The cis-form can be optically active.
4. Hexadentate Ligands
Complexes with a single hexadentate ligand like EDTA (e.g., $[Co(EDTA)]^-$) are generally Optically Active due to the twisted cage-like structure formed around the metal ion, lacking symmetry.
Practice Quiz
Test your ability to spot Chiral Complexes.
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