Question

Write the structures of a pair of complexes showing geometrical isomerism.

Answer 1

Geometrical isomerism arises in coordination compounds due to the different spatial arrangements of ligands around the central metal ion, especially in square planar and octahedral complexes.

It is not possible in tetrahedral complexes due to the equivalent positions of all ligands.

For example, the cis and trans forms of complex diamminedichloridoplatinum (II), [PtCl₂(NH₃)₂] are shown:

• Coordination number: 4
• Geometry: Square planar
• Ligands: 2 chloride ions (Cl⁻), 2 ammonia molecules (NH₃)

This complex exhibits geometrical isomerism because the two Cl⁻ ligands and two NH₃ ligands can occupy different positions around the central Pt²⁺ ion.

\[\begin{array}{cc}
\phantom{..}\ce{Cl \phantom{....}NH3}\\
\backslash\phantom{...}/\phantom{}\\
\ce{Pt}\\
/\phantom{...}\backslash\\
\ce{\underset{Cis}{\phantom{...}Cl\phantom{....}NH3}}
\end{array}\]     \[\begin{array}{cc}
\ce{H3N \phantom{....}Cl}\phantom{..}\\
\backslash\phantom{...}/\phantom{}\\
\ce{Pt}\\
/\phantom{...}\backslash\\
\ce{\underset{Trans}{\phantom{...}Cl \phantom{....}NH3}}
\end{array}\] 

The complex [PtCl₂(NH₃)₂] shows geometrical isomerism with cis and trans forms. These isomers have different spatial arrangements of ligands and different chemical and biological properties.