Two complexes [Cr(H2O)6]Cl3 (A) and [Cr(NH3)6]Cl3 (B) are violet and yellow coloured, respectively. The incorrect…

🧠 Identify the Wrong Statement

(A) $[\mathrm{Cr(H_2O)_6}]\mathrm{Cl_3}$ — violet. (B) $[\mathrm{Cr(NH_3)_6}]\mathrm{Cl_3}$ — yellow.

Cr(III) is d³ → $t_{2g}^3 e_g^0$ in both, with 3 unpaired electrons.

🗺️ Evaluate Each Statement

| # | Statement | Verdict | |---|---|---| | 1 | Both paramagnetic with 3 unpaired electrons | ✓ True (Cr(III) d³ has 3 unpaired) | | 2 | $\Delta_o$ of (A) < $\Delta_o$ of (B) | ✓ True (NH₃ > H₂O in spectrochemical series) | | 3 | $\Delta_o$ of (A), (B) calculated from energies of violet and yellow light, respectively | ✗ Incorrect | | 4 | Both absorb energies corresponding to their complementary colours | ✓ True |

🗺️ Why Statement 3 is Wrong

Observed colour = complement of absorbed colour (the absorbed light is what's missing from the transmitted/reflected spectrum).

• Observed violet → absorbs yellow light.
• Observed yellow → absorbs violet light.

So $\Delta_o$ of (A) is calculated from yellow light energy (not violet), and $\Delta_o$ of (B) from violet light energy (not yellow). Statement 3 has the mapping reversed.

🗺️ Sanity Check on $\Delta_o$ Order

NH₃ is stronger field than H₂O → bigger $\Delta_o$ for (B). Bigger $\Delta_o$ → absorbs higher-energy (shorter $\lambda$) photon → absorbs violet for (B), confirming (B) is yellow.

For (A) with H₂O (smaller $\Delta_o$): absorbs lower-energy (longer $\lambda$) photon → absorbs yellow, complement is violet. ✓

⚡ Colour Wheel Complements

| Absorbed | Observed | |---|---| | Red | Green | | Yellow | Violet | | Green | Red/pink | | Violet | Yellow | | Blue | Orange |

⚠️ The Crucial Distinction

$\Delta_o$ corresponds to absorbed light energy, NOT observed light energy. Always take the complement to find absorbed.

$\boxed{\text{Answer: (3) (the incorrect statement)}}$