JEE Chemistry 2025 Question with Solution

Given: The complexes are $[Co(NH_3)_5(H_2O)]^{3+}$, $[Co(NH_3)_6]^{3+}$, $[Co(CN)_6]^{3-}$ and $[CoCl(NH_3)_5]^{2+}$.

Find: The correct order in terms of wavelength of light absorbed.

Use crystal field theory and the spectrochemical series. Greater ligand field strength gives larger crystal field splitting $\Delta$. Since

we have larger $\Delta$ corresponding to higher absorbed energy and therefore smaller wavelength.

From the spectrochemical series:

So the ligand field strength increases in the order represented by the complexes as follows:

• $[CoCl(NH_3)_5]^{2+}$ has the weakest ligand set because of $Cl^-$.
• $[Co(NH_3)_5(H_2O)]^{3+}$ is stronger than D because $H_2O$ is stronger than $Cl^-$.
• $[Co(NH_3)_6]^{3+}$ has only $NH_3$ ligands, so splitting is larger.
• $[Co(CN)_6]^{3-}$ has $CN^-$, the strongest ligand here, so splitting is maximum.

Therefore, the crystal field splitting order is

Hence the wavelength absorbed, being inversely proportional to energy, is

Therefore, the correct option is A.

Given: Compare wavelength of absorbed light for the four cobalt complexes.

Find: The order of wavelength.

A quick route is to first compare ligand strength only. Since all complexes are compared through crystal field splitting, stronger ligand field means larger $\Delta$ and hence higher absorbed energy.

Use

and the ligand order

So the splitting order is immediately

Reverse this to get wavelength order:

Therefore, the correct option is A.