Stereoisomers are those isomers that have the same molecular formula and chemical bonds but they have different spatial arrangements of atoms. As already mentioned, stereoisomerism involves two types of isomerism viz., geometrical isomerism and optical isomerism. These are discussed below:
1. Geometrical isomerism
Geometrical isomerism arises in heteroleptic complexes due to ligands occupying different positions around the central ion. The ligands occupy positions either adjacent to one another or opposite to one another. These are referred to as cis-form (ligands occupy adjacent positions) and trans- form (ligands occupy opposite positions). This type of isomerism is, therefore, also referred to as cis-trans isomerism.
2. Optical isomerism
The isomers which rotate the plane of polarised light equally but in opposite directions are called optically active isomers. These are also called enantiomers or enantiomorphs. The d and l isomers are mirror images of each other just as left hand is mirror image of the right hand. These mirror image compounds are non-superimposable on each other and do not possess the plane of symmetry.
Which one of the following complexes exhibits optical isomerism?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → cis-[Co(NH3)2(C2O4)2]–
A complex shows optical isomerism when its mirror image cannot be superimposed on it.
In cis-\( \ce{[Co(NH3)2(C2O4)2]^-} \) the two oxalate ions (\( \ce{C2O4^2-} \), each didentate) and the two \( \ce{NH3} \) ligands in cis positions give a complex whose mirror image cannot be superimposed on it. It exists as a d and an l form, just like cis-\( \ce{[PtCl2(en)2]^2+} \).
Why the other options are incorrect
Option (1) trans-[Cr(NH3)4(SCN)2]+ — Incorrect. It has only unidentate ligands, and the two SCN groups lie opposite each other. Its mirror image can be superimposed on it, so the complex is optically inactive.
Option (3) trans-[Co(NH3)2(C2O4)2]– — Incorrect.
With the two \( \ce{NH3} \) ligands opposite each other, both oxalate rings lie in one plane through the metal.
The trans form can be superimposed on its mirror image and is optically inactive; as with \( \ce{[PtCl2(en)2]^2+} \), only the cis form is optically active.
Option (4) cis-[Pt(gly)2] — Incorrect. Pt(II) with four donor atoms is square planar. A flat complex can be superimposed on its mirror image, so it shows geometrical isomerism (cis and trans) but no optical isomerism.
NCERT (2026-27 edition):
5.4.2 Optical Isomerism
