Three complexes, [CoCl(NH3)5]2+ (I), [Co(NH3)5H2O]3+ (II) and [Co(NH3)6]3+ (III) absorb light in the visible region.…

🧠 λ ∝ 1/Δ

Larger field strength → larger Δ → shorter λ absorbed.

🗺️ Rank by Field Strength

| # | Complex | Field assessment | |---|---|---| | (I) | $[\mathrm{CoCl(NH_3)_5}]^{2+}$ | 5 NH₃ + 1 Cl⁻ (Cl⁻ weakens average field) — smallest Δ | | (II) | $[\mathrm{Co(NH_3)_5(H_2O)}]^{3+}$ | 5 NH₃ + 1 H₂O — moderate Δ | | (III) | $[\mathrm{Co(NH_3)_6}]^{3+}$ | 6 NH₃ — largest Δ |

So Δ: III > II > I, hence λ: III < II < I, i.e. I > II > III.

⚡ Spectrochemical Series Mini

For replacing one NH₃ ligand with one weaker ligand:

• → H₂O: small drop in Δ.
• → Cl⁻: bigger drop in Δ.
• → F⁻: similar to Cl⁻.

The relative position in the series: $\mathrm{Cl^- < F^- < H_2O < NH_3}$.

⚠️ Don't Forget Charge Effect

Going from $[\mathrm{Co(NH_3)_6}]^{3+}$ to $[\mathrm{CoCl(NH_3)_5}]^{2+}$ changes both ligand strength and complex charge (+3 → +2). Lower charge = weaker electrostatic = smaller Δ. Both effects act in the same direction here (both lower Δ for I), reinforcing the wavelength order.

$\boxed{\text{Answer: (3) (I) > (II) > (III)}}$