There are some deposits of nitrates and phosphates in the earth's crust. Nitrates are more soluble in water. Nitrates are difficult to reduce under laboratory conditions, but microbes do it easily. Ammonia, forms a large number of complexes with transition metals. Hybridization easily explains the ease of sigma donation capability of \(NH_3\) and \(PH_3\).
Phosphine is an inflammable gas, it is prepared from white phosphorus.
Ammonia forms complexes such as \([Cu(NH_3)_4]^{2+}\) with transition metal ions because:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → the nitrogen atom of \(NH_3\) has a lone pair of electrons which it donates to the metal ion
A ligand forms a complex by donating a lone pair of electrons to a vacant orbital of the metal ion, which makes a coordinate bond. The ligand is a Lewis base and the metal ion is a Lewis acid.
Option (1) the nitrogen atom of NH3 has a lone pair of electrons which it donates to the metal ion — Correct. The nitrogen of ammonia has one lone pair. It gives this pair to \( \ce{Cu^2+} \), so four \( \ce{NH3} \) molecules form the deep blue complex \( \ce{[Cu(NH3)4]^2+} \).
Option (2) NH3 is a Lewis acid which accepts electrons from the metal ion — Incorrect. \( \ce{NH3} \) is an electron-pair donor, that is, a Lewis base. The metal ion is the electron-pair acceptor.
Option (3) the nitrogen atom of NH3 has vacant d orbitals — Incorrect. Nitrogen is a second-period element and has no d orbitals. Complex formation needs a donor pair on the ligand, not vacant orbitals.
Option (4) NH3 is a strong reducing agent — Incorrect. Ammonia is only a mild reducing agent. Reducing power has nothing to do with forming a coordinate bond.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Properties – …
