JEE Main 2023 · 1 February · evening · Single correct · +4 / −1
The complex cation which has two isomers is :
- (a) [Co(NH₃)₅ Cl]⁺
- (b) [Co(H₂ O)₆]³⁺
- (c) [Co(NH₃)₅ NO₂]²⁺
- (d) [Co(NH₃)₅ Cl]²⁺
Answer and solution
Answer (c) [Co(NH₃)₅ NO₂]²⁺
Solution
Chemistry ma’am · 3 steps
With five identical ligands geometry gives nothing, so look for a ligand that can bind two ways.
- Geometry first. In an octahedral MA₅B or MA₆ complex, all six positions are equivalent. Wherever you put the single different ligand, you get the same complex, so there are no cis or trans forms.
- Ambidentate ligand. So the only route to a second isomer is a ligand that can bind through two different atoms. NO₂⁻ can bind through N (nitro, -NO₂) or through O (nitrito, -ONO). These two are linkage isomers.
Cation Type Geometrical isomers Ambidentate ligand [Co(NH₃)₅Cl]⁺ MA₅B none no, Cl⁻ binds one way [Co(H₂O)₆]³⁺ MA₆ none no [Co(NH₃)₅NO₂]²⁺ MA₅B none yes, N- or O-bonded [Co(NH₃)₅Cl]²⁺ MA₅B none no, Cl⁻ binds one way - So. Only (c) exists as two isomers: [Co(NH₃)₅(NO₂)]²⁺ (nitro) and [Co(NH₃)₅(ONO)]²⁺ (nitrito).
The idea to keep. An octahedral MA₅B never shows geometrical isomerism. If it is to have isomers, check for an ambidentate ligand like NO₂⁻, SCN⁻ or CN⁻ that gives linkage isomers.
Answer from the NTA final answer key. Solution by PrepWiser.