\(Ni^{2+}\) in \([NiCl_2(H_2O)_4]\) and \(Fe^{3+}\) in \([Fe(CN)_6]^{3-}\) are called:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Lewis acid
In a coordination entity the ligands donate electron pairs to the central metal atom or ion. In \( \ce{[NiCl2(H2O)4]} \), \( \ce{Ni^{2+}} \) accepts electron pairs from \( \ce{Cl^{-}} \) and \( \ce{H2O} \); in \( \ce{[Fe(CN)6]^{3-}} \), \( \ce{Fe^{3+}} \) accepts electron pairs from six \( \ce{CN^{-}} \) ions. An electron-pair acceptor is a Lewis acid, so these central ions are called Lewis acids (and the ligands are Lewis bases).
Why the other options are incorrect
Option (2) Lewis base — Incorrect. A Lewis base donates an electron pair; that is the role of the ligands, not the metal ions.
Option (3) Amphoteric compounds — Incorrect. The metal ions here only accept electron pairs; they are not acting both as acid and base.
Option (4) Ni2+ is acid, Fe3+ is base — Incorrect. Both metal ions accept electron pairs from their ligands, so both are Lewis acids.
NCERT (2026-27 edition):
Central Atom/Ion – “In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion. For example, the central atom/ion in the coordination entities: [NiCl2(H2O)4], [CoCl(NH3)5]2+ and [Fe(CN)6]3– are Ni2+, Co3+ and Fe3+, respectively. These central atoms/ions are also referred to as Lewis acids.”