The correct order of A, B, C and D in terms of wavenumber of light absorbed is: (A) [CoCl(NH3)5]2+ (B) [Co(CN)6]3- (C)…

🧠 Wavenumber ∝ $\Delta$

Higher field strength → larger $\Delta$ → photon needed to bridge $t_{2g}\to e_g$ has higher energy → higher wavenumber ($\bar\nu = 1/\lambda$).

So order of wavenumber = order of crystal-field splitting energy.

🗺️ Rank Each Complex by Total Field Strength

| # | Complex | Metal | Ligand mix | Field strength | |---|---|---|---|---| | (B) | $[\mathrm{Co(CN)_6}]^{3-}$ | Co(III) | 6 × strong CN⁻ | highest | | (C) | $[\mathrm{Co(NH_3)_5(H_2O)}]^{3+}$ | Co(III) | 5 NH₃ + 1 H₂O | high (NH₃ slightly > H₂O) | | (A) | $[\mathrm{CoCl(NH_3)_5}]^{2+}$ | Co(III) | 5 NH₃ + 1 Cl⁻ | moderate (Cl⁻ < H₂O) | | (D) | $[\mathrm{Cu(H_2O)_4}]^{2+}$ | Cu(II), d⁹ | 4 H₂O, distorted | lowest |

Order (low → high $\bar\nu$): D < A < C < B.

⚡ Why Cu(II) Is Lowest

Cu²⁺ has small $\Delta_o$ compared to 3+ Co(III), and its Jahn–Teller-distorted aquo complex absorbs in the red (long λ, low $\bar\nu$) → famously gives the pale blue solution. By contrast, $[\mathrm{Co(CN)_6}]^{3-}$ absorbs UV/violet → very high $\bar\nu$.

⚠️ Replacing One Strong-Field Ligand Matters

Going from [Co(NH₃)₆]³⁺ to [Co(NH₃)₅X]ⁿ⁺: each substitution shifts the average field. Replacing 1 NH₃ with 1 H₂O (C) or 1 Cl⁻ (A) lowers $\Delta$ — Cl⁻ lowers it more than H₂O. So A < C in $\Delta$ and in $\bar\nu$.

$\boxed{\text{Answer: (4) D < A < C < B}}$