Correct increasing order of wavelength of absorption in visible region for complex is:
A. \([Ni(CN)_6]^{4-}\)
B. \([Ni(H_2O)_6]^{2+}\)
C. \([Ni(CO)_6]^{2+}\)
D. \([Ni(NH_3)_6]^{2+}\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(C < A < D < B\)
In an octahedral complex the colour comes from a d–d transition whose energy equals the crystal field splitting Δo. Since E = hc/λ, a larger Δo means a shorter wavelength of light absorbed. All four complexes are of nickel with six ligands, so Δo follows the spectrochemical series (H2O < NH3 < CN− < CO):
C. [Ni(CO)6]2+ (first, shortest wavelength) — CO is the strongest field ligand, giving the largest Δo.
A. [Ni(CN)6]4− (second) — CN− is a strong field ligand, just below CO.
D. [Ni(NH3)6]2+ (third) — NH3 gives a smaller splitting than CN−.
B. [Ni(H2O)6]2+ (last, longest wavelength) — H2O is the weakest of the four, giving the smallest Δo.
Shortcut: CO (C) gives the largest splitting and the shortest wavelength; H2O (B) the smallest splitting and the longest. Options (2) and (4) run from C to B, and only Option (2) places the CN− complex A before the NH3 complex 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.”