The correct order of the complexes [Co(NH₃)₅(H₂ O)]³⁺(A),[Co(NH₃)₆]³⁺(B),[Co(CN)₆]³⁻(C) and [CoCl(NH₃)₅]²⁺(D) in terms of wavelength of light absorbed is

Solution

Chemistry ma’am · 4 steps

Rank the ligands by field strength to get Δo, then flip it: a bigger splitting absorbs a shorter wavelength.

1. Same metal throughout. All four complexes contain Co³⁺ (d⁶) in an octahedral field. So only the ligands change the splitting Δₒ.
2. Spectrochemical series. For these ligands the field strength order is Cl⁻ < H₂O < NH₃ < CN⁻. A and D each swap one NH₃ of B for a weaker ligand, and Cl⁻ is weaker than H₂O.

   Complex	Ligand set	Δₒ	Wavelength absorbed
   D	5 NH₃ + Cl⁻	smallest	longest
   A	5 NH₃ + H₂O	small	long
   B	6 NH₃	large	short
   C	6 CN⁻	largest	shortest

3. Energy to wavelength. The light absorbed has energy Δₒ = hc/λ, so λ is inversely related to Δₒ. The order of Δₒ reverses for wavelength.
4. So. Wavelength absorbed follows D > A > B > C, which is option (c).

The idea to keep. Strong-field ligands give a large Δₒ, and a large Δₒ means high-energy, short-wavelength light is absorbed. Always ask whether the question wants energy or wavelength.

Answer from the NTA final answer key. Solution by PrepWiser.