Question

Discuss the optical isomerism shown by the complex of the type [M(AA)₂a₂]^n±. Give suitable examples.

Answer 1

In coordination complexes of the type [M(AA)₂a₂]^n± (where M is the central metal ion, AA represents a symmetrical bidentate ligand, and ‘a’ represents a monodentate ligand), optical isomerism is observed only in the cis isomer. These complexes have an octahedral geometry because the two bidentate ligands occupy four coordination sites and the two monodentate ligands occupy the remaining two, resulting in a total coordination number of 6.

• Cis isomer: In the cis arrangement, the two monodentate ligands (‘a’) are adjacent to each other. This arrangement lacks a plane of symmetry, making the cis isomer chiral and thus optically active. The cis isomer exists as a pair of enantiomers (non-superimposable mirror images) referred to as dextro (d) and levo (l) forms.
• Trans isomer: In the trans arrangement, the two monodentate ligands (‘a’) are positioned opposite to each other. This arrangement possesses a plane of symmetry, rendering the trans isomer achiral and therefore optically inactive.

For example, [Co(en)₂Cl₂]⁺ it is a classic example, where ‘en’ (ethylenediamine) is the bidentate ligand and ‘Cl’ (chloride) is the monodentate ligand.

• cis-[Co(en)₂Cl₂]⁺: The cis-isomer is optically active and exists as two enantiomers (d and l forms).
• trans-[Co(en)₂Cl₂]⁺: The trans-isomer is optically inactive.