Geometrical Isomerism
Stereoisomerism in Coordination Compounds
1. What is Geometrical Isomerism?
Definition: It arises in heteroleptic complexes due to different possible geometric arrangements of the ligands around the central metal atom.
Commonly known as Cis-Trans Isomerism.
- Cis-isomer: Identical ligands occupy adjacent positions.
- Trans-isomer: Identical ligands occupy opposite positions.
2. Coordination Number 4
A. Tetrahedral Complexes
Tetrahedral complexes ($sp^3$) do not show geometrical isomerism because all four positions are adjacent and equivalent to each other relative to the central metal atom.
B. Square Planar Complexes
Shown by complexes of type $MA_2B_2$, $MA_2BC$, and $MABCD$.
Type $MA_2B_2$ Example: $[Pt(NH_3)_2Cl_2]$
| Isomer | Description | Property |
|---|---|---|
| Cis-platin | $Cl$ atoms are adjacent ($90^\circ$) | Used as Anti-cancer drug |
| Trans-platin | $Cl$ atoms are opposite ($180^\circ$) | Therapeutically inactive |
3. Coordination Number 6 (Octahedral)
A. Type $MA_4B_2$
Example: $[Co(NH_3)_4Cl_2]^+$
- Cis: The two $B$ ligands (Cl) are at adjacent positions (e.g., positions 1,2 or 2,3).
- Trans: The two $B$ ligands are at opposite positions (e.g., positions 1,6).
B. Type $MA_3B_3$ (Fac-Mer Isomerism)
Example: $[Co(NH_3)_3(NO_2)_3]$
- Facial (Fac): Three identical ligands occupy three adjacent corners of an octahedral face. (cis-like).
- Meridional (Mer): Three identical ligands occupy positions around the meridian of the octahedron. (trans-like).
C. With Bidentate Ligands: $M(AA)_2B_2$
Example: $[Co(en)_2Cl_2]^+$ (where $en$ = ethylenediamine)
- Cis: Chlorine atoms are adjacent. This form is Optically Active.
- Trans: Chlorine atoms are opposite. This form is Optically Inactive (has plane of symmetry).
Practice Quiz
Test your knowledge on Coordination Isomerism.
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