Geometrical isomerism in complex with coordination number four

Geometrical Isomerism in Complexes with Coordination Number Four

Geometrical isomerism is a form of stereoisomerism that results from the distinct positioning of ligands around the central metal atom or ion in a complex. In complexes with a coordination number of four, this type of isomerism is commonly observed when the geometry is square planar. It is rarely seen in tetrahedral complexes.

Square Planar Geometry

In square planar complexes, the central metal ion lies at the center of a square, with four ligands positioned at the corners. Geometrical isomerism arises when similar ligands are placed next to each other (cis) or across from each other (trans).

Cis-Trans Isomerism in Square Planar Complexes

• Cis-isomer: Two identical ligands are placed next to each other (90° apart).
• Trans-isomer: The same ligands are placed opposite each other (180° apart).

Example: [Pt(NH₃)₂Cl₂]

• In the cis-isomer, both Cl ligands are adjacent, and both NH₃ are adjacent.
• In the trans-isomer, Cl ligands are opposite each other, and NH₃ are also opposite.

Tetrahedral Geometry

Geometrical isomerism is not observed in tetrahedral complexes with four ligands because all positions are equivalent in space. Therefore, no distinct cis or trans forms can exist.

Key Points

• Geometrical isomerism is common in square planar complexes of the type Ma₂b₂.
• Tetrahedral complexes do not exhibit geometrical isomerism due to symmetrical ligand positions.