Arrange the following ligands in decreasing order of field strength in spectrochemical series.
(A) \( Br^- \)
(B) \( H_2O \)
(C) \( OH^- \)
(D) \( CO \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (D), (B), (C), (A)
The ability of a ligand to split the d orbitals of a metal ion is its field strength. Ligands arranged in increasing order of field strength form the spectrochemical series, which is found experimentally from the light absorbed by their complexes. In this series halide ions come at the weak end, \( \ce{OH-} \) and \( \ce{H2O} \) in the middle, and CO at the strong end.
(A) \( \ce{Br-} \): near the weak end, only \( \ce{I-} \) is weaker.
(B) \( \ce{H2O} \): a moderate ligand, stronger than \( \ce{OH-} \).
(C) \( \ce{OH-} \): weaker than water although it is charged.
(D) CO: the strongest ligand of the series; it also accepts electron density from the metal into its \( \pi^{*} \) orbitals.
In decreasing order of field strength:
\( \displaystyle \ce{CO} \gt \ce{H2O} \gt \ce{OH-} \gt \ce{Br-} \)
Shortcut: CO is the strongest ligand of the whole series, and only Option (1) starts with (D).
NCERT (2026-27 edition):
Crystal Field Theory – “… In general, ligands can be arranged in a series in the order of increasing field strength as given below:
I– < Br– < SCN– < Cl– < S2– < F– < OH– < C2O42– < H2O < NCS– < edta4– < NH3 < en < CN– < CO
Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands.”