Question

Three complexes ${\left[\mathrm{CoCl}{\left({\mathrm{NH}}_3\right)}_5\right]}^{+2}\left(1\right),{\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{H}}_2\mathrm{O}\right]}^{3+}\left(\mathrm{II}\right)$ and ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}\left(\mathrm{III}\right)$absorb light in the visible region. The correct order of the wavelengths of the light absorbed by them is,

• A. $\left(\mathrm{III}\right)>\left(\mathrm{I}\right)>\left(\mathrm{II}\right)$
• B. $\left(\mathrm{I}\right)>\left(\mathrm{II}\right)>\left(\mathrm{III}\right)$
• C. $\left(\mathrm{II}\right)>\left(\mathrm{I}\right)>\left(\mathrm{III}\right)$
• D. $\left(\mathrm{III}\right)>\left(\mathrm{II}\right)>\left(\mathrm{I}\right)$

Answer 1

The correct option is B

$\left(\mathrm{I}\right)>\left(\mathrm{II}\right)>\left(\mathrm{III}\right)$

The explanation for the correct option:

In case of option B: $\left(\mathrm{I}\right)>\left(\mathrm{II}\right)>\left(\mathrm{III}\right)$

1. A compound with a strong field ligand has a proclivity for absorbing high-energy photons.
2. $\{\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6{\}}^{3+}$ absorbs the most energy and has the shortest wavelength among the three complexes.
3. The field of ${\left\{\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_5{\mathrm{H}}_2\mathrm{O}\right\}}^{3+}$ is weaker than that of the aforementioned compound, therefore it absorbs less energy and higher wavelengths.
4. $[\mathrm{CoCl}{\left({\mathrm{NH}}_3\right)}_5{]}^{2+}$ has the weakest field, and therefore will absorb light of least energy and highest wavelength.
5. The strength of the ligand ${\mathrm{NH}}_3>{\mathrm{H}}_2\mathrm{O}>{\mathrm{Cl}}^{-}$.

Hence, option B is the correct option.