Arrange following ligands in their increasing order of field strength
(A) NCS–
(B) OH–
(C) edta4–
(D) en
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B), (A), (C), (D)
The field strength of a ligand is its ability to split the d orbitals of the metal ion (the crystal field splitting Δo). Arranged in increasing order of field strength, found from the absorption spectra of their complexes, the ligands form the spectrochemical series. In it, thiocyanate bonded through nitrogen (NCS−) is a much stronger ligand than when bonded through sulphur (\( \ce{SCN^{-}} \)).
Picking out the four given ligands in that series:
(B) \( \ce{OH^{-}} \): oxygen donor, weakest of the four
(A) \( \ce{NCS^{-}} \): N-bonded thiocyanate
(C) edta4−: hexadentate, two N and four O donors
(D) en: ethane-1,2-diamine, two N donors, strongest of the four
Shortcut: \( \ce{OH^{-}} \) (B) is the weakest and en (D) the strongest, which leaves Options (1) and (3). \( \ce{NCS^{-}} \) comes before edta4− only in Option (1).
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.”