Which of the following ligands have weakest field strength?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → SCN−
Ligands can be arranged by the size of the crystal field splitting they cause. This experimentally found order is the spectrochemical series. Ligands at the start of the series are weak field ligands and those at the end are strong field ligands. Of the four given ligands, SCN− (bonded through sulphur) comes first, before \( \ce{F^{-}} \), \( \ce{OH^{-}} \) and \( \ce{NCS^{-}} \), so it has the weakest field strength.
Why the other options are incorrect
Option (2) F− — Incorrect. It comes later in the series than \( \ce{SCN^{-}} \), so it is a stronger field ligand.
Option (3) NCS− — Incorrect. The same ion bonded through nitrogen is a much stronger ligand, even stronger than \( \ce{H2O} \).
Option (4) OH− — Incorrect. It comes later in the series than \( \ce{SCN^{-}} \) and \( \ce{F^{-}} \), so it is a stronger field ligand.
NCERT (2026-27 edition):
Crystal Field Theory – …
