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.
Consider the following statements about stereoisomerism in coordination compounds.
(A) Geometrical isomerism also arises in complexes of the formula \( \ce{[MX2(L-L)2]} \), where L–L is a didentate ligand such as ethane-1,2-diamine.
(B) Important examples of geometrical isomerism are found with coordination numbers 2 and 3.
(C) Molecules or ions that cannot be superimposed on their mirror images are called chiral.
(D) In a polarimeter, the dextro (d) form of an optical isomer rotates the plane of polarised light to the left and the laevo (l) form rotates it to the right.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
Geometrical isomers differ in whether like ligands sit next to each other (cis) or opposite each other (trans). Optical isomers are mirror images that cannot be placed one on top of the other.
(A) Geometrical isomerism also arises in complexes of the formula \( \ce{[MX2(L-L)2]} \), where L–L is a didentate ligand such as ethane-1,2-diamine. — True. The two X ligands can still be adjacent (cis) or opposite (trans) to each other, even though each en ligand takes up two positions.
(B) Important examples of geometrical isomerism are found with coordination numbers 2 and 3. — False. The important examples are found with coordination numbers 4 and 6 (square planar and octahedral complexes).
(C) Molecules or ions that cannot be superimposed on their mirror images are called chiral. — True. Such a molecule or ion is chiral, and its two mirror-image forms are the d and l enantiomers.
(D) In a polarimeter, the dextro (d) form of an optical isomer rotates the plane of polarised light to the left and the laevo (l) form rotates it to the right. — False. The directions are the other way round: the d form rotates the plane to the right and the l form to the left.
NCERT (2026-27 edition):
5.4.1 Geometric Isomerism – “…Important examples of this behaviour are found with coordination numbers 4 and 6.”
“This type of isomerism also arises when didentate ligands L – L [e.g., NH2CH2CH2NH2 (en)] are present in complexes of formula [MX2(L – L)2] …”
5.4.2 Optical Isomerism – “Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left).”