Geometrical Isomerism in Square Planar Complexes
Exploring cis-trans isomerism in 4-coordinate $dsp^2$ systems.
1. Why Square Planar?
Coordination compounds with a Coordination Number of 4 can adopt either a tetrahedral or a square planar geometry. Tetrahedral complexes do not show geometrical isomerism because all four positions are adjacent to one another (bond angles are all $109.5^\circ$).
However, in square planar complexes ($dsp^2$ hybridization, commonly seen in $d^8$ metal ions like $Pt^{2+}$, $Pd^{2+}$, and $Ni^{2+}$), the ligands lie in the same plane. Two ligands can either be adjacent to each other (at a $90^\circ$ angle) or opposite to each other (at a $180^\circ$ angle). This difference allows for geometrical isomerism.
2. The $MA_2B_2$ Type: Cis and Trans
The most famous example of square planar geometrical isomerism belongs to the general formula $MA_2B_2$.
- Cis-isomer: The two identical ligands occupy adjacent positions ($90^\circ$ to each other).
- Trans-isomer: The two identical ligands occupy opposite positions ($180^\circ$ to each other).
Cis-platin
cis-$[Pt(NH_3)_2Cl_2]$
Identical ligands are at $90^\circ$ angles.
Trans-platin
trans-$[Pt(NH_3)_2Cl_2]$
Identical ligands are opposite ($180^\circ$).
3. Other General Types
Geometrical isomerism can occur in several other formulations of square planar complexes:
Shows 2 isomers (cis and trans). The classification is based on the orientation of the two identical 'A' ligands relative to each other.
Shows 3 isomers. You can find these by fixing the position of ligand 'A' and placing the other three ligands (B, C, and D) in the position trans to 'A' one by one.
Where $(AB)$ is an unsymmetrical bidentate ligand (like glycinato, $gly^-$). This complex can exist in cis and trans forms depending on whether identical donor atoms are adjacent or opposite. Note: Complexes with symmetrical bidentate ligands $M(AA)_2$ do NOT show geometrical isomerism.
Test Your Knowledge
Practice MCQs on Square Planar Isomerism
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