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.
A similarity between optical and geometrical isomerism is that
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → both are included in the stereoisomerism
The passage states that “stereoisomerism involves two types of isomerism viz., geometrical isomerism and optical isomerism”.
Stereoisomers have the same molecular formula and the same chemical bonds but a different arrangement of atoms in space. Cis and trans isomers differ in where like ligands sit; d and l isomers are non-superimposable mirror images. Both are therefore kinds of stereoisomerism, and that is what they share.
Why the other options are incorrect
Option (1) each gives an equal number of isomers for a given compound — Incorrect. The counts differ. Square planar \( \ce{[Pt(NH3)2Cl2]} \) has two geometrical isomers but no optical isomer.
Option (2) if in a compound one is present, then so is the other — Incorrect. Each can occur alone. \( \ce{[Pt(NH3)2Cl2]} \) shows only geometrical isomerism, while \( \ce{[Co(en)3]^3+} \) shows only optical isomerism, since it has three identical didentate ligands and no cis-trans forms.
Option (4) they have no similarity — Incorrect. Both arise from a different spatial arrangement of the same atoms and bonds, so they are two types of the same isomerism.