Which among the following is a strong field ligand ?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → NH3
Ligands arranged in order of increasing field strength form the spectrochemical series. The four given ligands appear in it as \( \ce{I-} \) < \( \ce{SCN-} \) < \( \ce{Cl-} \) < … < \( \ce{NH3} \).
Of the four options, \( \ce{NH3} \) stands highest in the series, close to the strong field end; it forms low spin complexes with ions such as \( \ce{Co^3+} \) (\( \ce{[Co(NH3)6]^3+} \) is diamagnetic). So the strong field ligand is NH3.
Why the other options are incorrect
Option (1) I− — Incorrect. The weakest field ligand of the series.
Option (2) Cl− — Incorrect. A weak field ligand.
Option (4) SCN− — Incorrect. Bonded through sulphur it is a weak field ligand, near \( \ce{Br-} \).
NCERT (2026-27 edition):
Crystal Field Theory – …
