Question

The colour of the coordination compounds depends on the crystal field splitting. What will be the correct order of absorption of wavelength of light in the visible region, for the complexes, $$[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}$$, $$[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$, $$[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}$$

Options

• $$[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}>[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}$$

• $$[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$

• $$[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$

• $$[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}>[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}$$

Answer 1

$$[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$

Explanation:

The CFSE of the ligands is in the order:

H₂O < NH₃ < CN⁻

Hence, excitation energies are in the order:

$$[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}<[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}<[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$

From the relation E = ${\displaystyle \frac{\text{hc}}{\lambda }}$ ⇒ ${\displaystyle \text{E}\propto \frac{1}{\lambda }}$

The order of absorption of the wavelength of light in the visible region is:

$$[\mathrm{Co}(\mathrm{H}{\phantom{A}}_2\mathrm{O}){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{NH}{\phantom{A}}_3){\phantom{A}}_6]{\phantom{A}}^{3+}>[\mathrm{Co}(\mathrm{CN}){\phantom{A}}_6]{\phantom{A}}^{3-}$$