Arrange the following complexes in decreasing order of absorption of wavelength of light in the visible region:

(A) $[Co(CN)_6]^{3-}$
(B) $[CoF_6]^{3-}$
(C) $[Co(H_2O)_6]^{3+}$
(D) $[Co(NH_3)_6]^{3+}$

Choose the correct option from below:

(B), (C), (D), (A)

The correct answer is Option (1) → (B), (C), (D), (A)

In coordination compounds, absorption of visible light occurs due to d–d electronic transitions. The energy required for this transition depends on the crystal field splitting energy (Δ₀).

Relationship between wavelength and crystal field splitting: Δ₀ ∝ 1 / λ

Thus:

• Larger splitting energy → smaller wavelength absorbed

• Smaller splitting energy → larger wavelength absorbed

Therefore, to find the decreasing order of wavelength absorbed, we must arrange the complexes in increasing order of crystal field splitting.

Ligand strength (Spectrochemical series)

Weak field ligands → small splitting

Strong field ligands → large splitting

Relevant order from the spectrochemical series: F⁻ < H₂O < NH₃ < CN⁻

Thus crystal field splitting increases in the order: F⁻ < H₂O < NH₃ < CN⁻

(B) [CoF₆]³⁻ → weakest ligand → smallest splitting → largest wavelength absorbed

(C) [Co(H₂O)₆]³⁺ → slightly stronger ligand

(D) [Co(NH₃)₆]³⁺ → stronger ligand

(A) [Co(CN)₆]³⁻ → strongest ligand → largest splitting → smallest wavelength absorbed

Decreasing order of wavelength absorbed (B) > (C) > (D) > (A)