Question

The correct increasing order for the wavelengths of absorption in the visible region for the following Co$^{3+}$ complexes is:
(i) [Co(NH$_3$)$_6$]$^{3+}$
(ii) [Co(H$_2$O)$_6$]$^{3+}$
(iii) [Co(en)$_3$]$^{3+}$

• A. iii, i, ii
• B. i, ii, iii
• C. ii, i, iii
• D. iii, ii, i

Hint:

Stronger ligands cause greater d-d splitting, leading to lower wavelength absorption.

Answer 1

The Correct Option is A

Wavelength $\propto \dfrac{1}{\Delta}$ (Crystal field splitting energy)
Order of ligand strength: H$_2$O<NH$_3$<en
Thus, $\Delta$: (ii)<(i)<(iii)
So, $\lambda$: (iii)<(i)<(ii) (inverse relation)