Assertion (A): Complexes of a metal ion with ethane-1,2-diamine tend to be more stable than similar complexes of the same metal ion with ammonia.
Reason (R): Ethane-1,2-diamine can use its two donor nitrogen atoms simultaneously to bind a single metal ion, and so acts as a chelate ligand.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Ammonia is a unidentate ligand: it binds through its single nitrogen atom. Ethane-1,2-diamine, H2NCH2CH2NH2 (en), is didentate: it has two nitrogen donor atoms.
Assertion (A): Complexes of a metal ion with ethane-1,2-diamine tend to be more stable than similar complexes of the same metal ion with ammonia. — True. Both ligands bind through nitrogen, but the complexes of en are chelate complexes, and chelate complexes tend to be more stable than similar complexes of unidentate ligands.
Reason (R): Ethane-1,2-diamine can use its two donor nitrogen atoms simultaneously to bind a single metal ion, and so acts as a chelate ligand. — True. A di- or polydentate ligand that uses two or more donor atoms at the same time on one metal ion is a chelate ligand.
Why R explains A?
Assertion (A) compares the stability of the two kinds of complexes; Reason (R) states what makes en different from NH3.
Because en holds the metal ion through two donor atoms at once, its complexes are chelate complexes, and this is the cause of their greater stability.
NCERT (2026-27 edition):
5.2 (c) Ligands – “When a ligand is bound to a metal ion through a single donor atom, as with Cl–, H2O or NH3, the ligand is said to be unidentate. When a ligand can bind through two donor atoms as in H2NCH2CH2NH2 (ethane-1,2-diamine) or C2O42– (oxalate), the ligand is said to be didentate…”
“When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. Such complexes, called chelate complexes tend to be more stable than similar complexes containing unidentate ligands.”