Chemistry - Coordination Chemistry Question with Solution | TestHub

The color of coordination compounds depends on the ligand field strength. H₂O is a weak field ligand, en (ethylenediamine) is a strong field ligand, and CN⁻ is a very strong field ligand.

Ligand field strength order: H₂O < en < CN⁻

Complex I: $[\mathrm{M}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{+3}$ (green) - weak field

Complex III: $[\mathrm{M}(\mathrm{CN}{)}_6{]}^{-3}$ (red) - very strong field

Since en is a stronger field ligand than H₂O but weaker than CN⁻, the crystal field splitting energy (${\Delta }_o$) for complex II, $[\mathrm{M}(\mathrm{en}{)}_3{]}^{+3}$, will be intermediate. This leads to absorption of light with higher energy than complex I but lower than complex III.

The complementary color of green is red, and the complementary color of red is green.

If complex I (weak field) is green, it absorbs red light.

If complex III (very strong field) is red, it absorbs green light.

Complex II will absorb light of higher energy than red but lower than green. This corresponds to absorbing yellow/orange light, making the complex appear violet.

Therefore, the correct matching color for complex II is violet.

The final answer is $\boxed{A}$