Consider the following complexes
(A) [CoCl(NH₃)₅]²⁺, (B) [Co(CN)₆]³⁻, (C) [Co(NH₃)₅(H₂ O)]³⁺ , (D) [Cu(H₂ O)₄]²⁺
The correct order of A, B, C and D in terms of wavenumber of light absorbed is :

Solution

Chemistry ma’am · 3 steps

Wavenumber rises with the splitting energy, so rank by metal charge first and ligand strength next.

1. What is measured. The light absorbed lifts a d electron across the crystal field gap, so its energy equals Δ. Energy is proportional to wavenumber, E = hcν̄, so a larger Δ means a larger wavenumber.
2. What sets the gap. A higher charge on the metal ion gives a larger Δ. Stronger-field ligands also give a larger Δ, in the order Cl⁻ < H₂O < NH₃ < CN⁻.

   Complex	Metal ion	Ligands	Splitting
   (D) [Cu(H₂O)₄]²⁺	Cu²⁺	4 weak H₂O	smallest: lower charge, weak ligand
   (A) [CoCl(NH₃)₅]²⁺	Co³⁺	5 NH₃ + 1 Cl⁻	larger; Cl⁻ is the weakest ligand
   (C) [Co(NH₃)₅(H₂O)]³⁺	Co³⁺	5 NH₃ + 1 H₂O	a little larger than A, since H₂O > Cl⁻
   (B) [Co(CN)₆]³⁻	Co³⁺	6 strong CN⁻	largest: strongest-field ligand

3. So. The splitting, and so the wavenumber absorbed, rises as D < A < C < B. That is option (a).

The idea to keep. Bigger Δ means higher energy, higher wavenumber and shorter wavelength. Compare the metal's oxidation state first, then the ligands using the spectrochemical series.

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