The correct order of energy of absorption for the following metal complexes is: A: [Ni(en)3]2+, B: [Ni(NH3)6]2+, C:…

🧠 All Three Are Ni(II) — Compare Ligands

Same metal (Ni²⁺ d⁸), same coord number, same geometry (octahedral). So energy of absorption tracks $\Delta_o$, which tracks ligand field strength.

🗺️ Spectrochemical Series

$\mathrm{H_2O < NH_3 < en}$

(en = ethylenediamine, a bidentate amine — slightly stronger than monodentate NH₃ due to the chelate effect.)

So $\Delta_o$ ranks: C ($\mathrm{H_2O}$) < B (NH₃) < A (en).

Energy of absorbed light $E = h\nu = hc/\lambda \propto \Delta_o$.

Therefore: C < B < A.

⚡ Why en > NH₃

en is a bidentate ligand: each en bridges two adjacent coordination sites with two N–donor atoms. This adds:

• a slight inductive boost in σ-donation,
• the chelate stabilisation entropically lowers the system energy,

both of which marginally increase Δ. The effect is small but real, and JEE consistently puts en above NH₃ in the spectrochemical series.

⚠️ Don't Conflate "Higher Δ" with "Stronger Bond"

A larger $\Delta_o$ doesn't necessarily mean stronger M–L bond strength — it means a larger gap between $t_{2g}$ and $e_g$. The bond strength itself depends on σ + π components together, which CFT doesn't track.

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