Arrange the following ligands in increasing order of their field strength:
(A) \( SCN^- \)
(B) \( F^- \)
(C) \( H_2O \)
(D) \( CO \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (C), (D)
The field strength of a ligand is its ability to split the d orbitals of the metal ion; arranged in order of increasing splitting, the ligands form the spectrochemical series.
(A) \( \ce{SCN-} \): bonded through sulphur, a weak field ligand; the weakest of the four, so it comes first.
(B) \( \ce{F-} \): stronger than \( \ce{SCN-} \) but weaker than water; second.
(C) \( \ce{H2O} \): above \( \ce{F-} \) in the series; third.
(D) CO: it also accepts electron density back from the metal, which makes it the strongest field ligand; last.
Shortcut: CO is the strongest ligand of the series, so the order ends with (D): Options (2) and (3). \( \ce{SCN-} \) is the weakest of the four, so it comes first, which leaves Option (3).
NCERT (2026-27 edition):
Crystal Field Theory – …
