\([Cu(en)_2]^{2+}\) is more stable than \([Cu(NH_3)_4]^{2+}\) because
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Ligand is didentate and forms a chelate complex
Ethane-1,2-diamine (en) binds the \( \ce{Cu^{2+}} \) ion through both nitrogen atoms, forming stable five-membered rings. Such chelate complexes are more stable than similar complexes of unidentate ligands (the chelate effect). So \( \ce{[Cu(en)2]^{2+}} \), with two chelate rings, is more stable than \( \ce{[Cu(NH3)4]^{2+}} \), in which each \( \ce{NH3} \) binds through one nitrogen.
Why the other options are incorrect
Option (1) It is a heteroleptic complex — Incorrect. \( \ce{[Cu(en)2]^{2+}} \) has only one kind of ligand, so it is homoleptic, not heteroleptic.
Option (2) Ethylene diamine has an alkyl group — Incorrect. The extra stability comes from ring formation through two donor atoms, not from the presence of a \( \ce{CH2-CH2} \) (alkyl) group.
Option (3) Copper in +2 oxidation state is more stable — Incorrect. Copper is in the +2 state in both complexes, so this cannot explain a difference between them.
NCERT (2026-27 edition):
Ligands – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 02-06 S3 31-05 S1 30-05 S2 | – |