Question

Discuss the optical isomerism shown by the complex of the type [M(AA)a₂b₂]. Give suitable examples.

Answer 1

Complexes of the type [M(AA)a₂b₂] (where M is a central metal atom, AA is a symmetrical bidentate ligand, and ‘a’ and ‘b’ are monodentate ligands) are octahedral and can exhibit both geometrical and optical isomerism.

• Cis isomers: When the two ‘a’ ligands and the two ‘b’ ligands are in cis positions, meaning they are adjacent to each other, the resulting geometry can be chiral and exhibit optical isomerism. The cis isomer exists as a pair of enantiomers.
• Trans isomers: When the two ‘a’ ligands or the two ‘b’ ligands are in trans positions, the complex has a plane of symmetry and is therefore optically inactive.

For example, [Co(en)(NH₃)₂Cl₂]⁺ in this complex has three geometrical isomers. This is a good example of a complex with this general formula where “en” represents ethylenediamine (a bidentate ligand), and NH₃ and Cl represent monodentate ligands.

• cis-[Co(en)(NH₃)₂Cl₂]⁺: This isomer is optically active and exists as two enantiomeric forms (d and l forms).
• trans-[Co(en)(NH₃)₂Cl₂]⁺: This isomer is optically inactive due to the presence of a plane of symmetry.