The correct order of the complexes [Co(NH3)5(H2O)]3+ (A), [Co(NH3)6]3+ (B), [Co(CN)6]3- (C) and [CoCl(NH3)5]2+ (D) in…

🧠 λ Inverse to Δₒ; Rank by Field Strength

Bigger $\Delta_o$ → shorter wavelength absorbed. Order $\lambda$ inversely to ligand field strength.

🗺️ Rank by Δₒ (Ligand Strength)

| # | Complex | Ligand average | Δₒ rank | |---|---|---|---| | (D) | $[\mathrm{CoCl(NH_3)_5}]^{2+}$ | 5 NH₃ + 1 Cl⁻ (Cl⁻ weakest) | smallest Δₒ | | (A) | $[\mathrm{Co(NH_3)_5(H_2O)}]^{3+}$ | 5 NH₃ + 1 H₂O | second smallest | | (B) | $[\mathrm{Co(NH_3)_6}]^{3+}$ | 6 NH₃ | larger | | (C) | $[\mathrm{Co(CN)_6}]^{3-}$ | 6 CN⁻ (strongest) | largest Δₒ |

So Δₒ: C > B > A > D.

🗺️ Convert to λ Order

λ ∝ 1/Δₒ → λ: D > A > B > C.

⚡ Spectrochemical Series Mini

For Co(III) complexes, key positions: $\mathrm{Cl^- < H_2O < NH_3 < CN^-}$

Replacing one NH₃ with weaker ligand drops $\Delta_o$; replacing all NH₃ with CN⁻ raises $\Delta_o$ dramatically.

⚠️ Charge Effects Layer on Top

Going from $[\mathrm{Co(NH_3)_6}]^{3+}$ (charge +3) to $[\mathrm{CoCl(NH_3)_5}]^{2+}$ (charge +2): both ligand-strength weakening and charge reduction lower $\Delta_o$. Both effects reinforce here.

$\boxed{\text{Answer: (1) D > A > B > C}}$