JEE Main 2024 · 9 April · morning · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The total number of geometrical isomers shown by [Co(en)₂ Cl₂]⁺ complex ion is three.
Reason (R): [Co(en)₂ Cl₂]⁺ complex ion has an octahedral geometry.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) (A) is correct but (R) is not correct
- (b) (A) is not correct but (R) is correct
- (c) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (d) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Answer and solution
Answer (b) (A) is not correct but (R) is correct
Solution
Chemistry ma’am · 4 steps
Two geometrical isomers, cis and trans; the third isomer is the cis mirror image, an optical isomer.
- Check the Reason. Each en is bidentate and takes 2 positions. With two Cl, that makes 2 × 2 + 2 = 6 donor atoms around Co³⁺, so the ion is octahedral. (R) is correct.
- Geometrical isomers. This is an M(AA)₂B₂ type complex. The two Cl ligands can sit next to each other at 90° (cis) or opposite each other at 180° (trans). There is no third arrangement, so there are only two geometrical isomers.
- Where three comes from. The cis isomer has no plane of symmetry, so it exists as a pair of non-superimposable mirror images. The trans isomer has a plane of symmetry and is optically inactive. That gives 2 + 1 = 3 stereoisomers in total, but only 2 of them are geometrical isomers.
- So. (A) is wrong because it counts an optical isomer as a geometrical one, and (R) is right. The answer is (b).
The idea to keep. For M(AA)₂B₂, count geometrical isomers first (cis and trans), then ask which ones are chiral. The cis isomer splits into two enantiomers, giving 3 stereoisomers but still only 2 geometrical isomers.
Answer from the NTA final answer key. Solution by PrepWiser.