Identify the two complexes and oxidation states

The complexes are: $[\mathrm{CoCl}(\mathrm{NH_3})_5]^{2+}$ and $[\mathrm{Co}(\mathrm{NH_3})_5(\mathrm{H_2O})]^{3+}$

Let us find the oxidation state of Co in each.

• For $[\mathrm{CoCl}(\mathrm{NH_3})_5]^{2+}$: $x+(-1)+5(0)=+2 \Rightarrow x=+3$ So cobalt is in the $+3$ oxidation state.

• For $[\mathrm{Co}(\mathrm{NH_3})_5(\mathrm{H_2O})]^{3+}$: $x+5(0)+0=+3 \Rightarrow x=+3$ So cobalt is also in the $+3$ oxidation state.

Check Assertion A

Assertion says: $[\mathrm{CoCl}(\mathrm{NH_3})_5]^{2+}$ absorbs at lower wavelength than $[\mathrm{Co}(\mathrm{NH_3})_5(\mathrm{H_2O})]^{3+}$

Absorbed wavelength is inversely proportional to crystal field splitting: $\Delta \propto \frac{1}{\lambda}$

So lower wavelength means larger $\Delta$.

Now compare ligand field strengths:

• $\mathrm{Cl^-}$ is a weak field ligand
• $\mathrm{H_2O}$ is stronger than $\mathrm{Cl^-}$
• $\mathrm{NH_3}$ is stronger still, but both complexes already have five $\mathrm{NH_3}$ ligands

Therefore, $[\mathrm{Co}(\mathrm{NH_3})_5(\mathrm{H_2O})]^{3+}$ should have larger splitting than $[\mathrm{CoCl}(\mathrm{NH_3})_5]^{2+}$ because $\mathrm{H_2O}$ is a stronger ligand than $\mathrm{Cl^-}$.

Hence it will absorb higher energy and therefore lower wavelength light.

So the assertion is false.

Check Reason R

Reason says: "It is because the wavelength of the light absorbed depends on the oxidation state of the metal ion."

This statement is generally true. In coordination compounds, the crystal field splitting depends on factors including:

• nature of ligand
• oxidation state of metal ion
• identity of metal ion
• geometry

Higher oxidation state usually increases $\Delta$, affecting the absorbed wavelength.

So the reason is true as a general statement.

Match with options

• Assertion A: False
• Reason R: True

Therefore the correct option is: $\boxed{\text{A}}$

Compare with stored answer

Stored correct answer: A

My derived answer matches the stored answer.