JEE Advanced 2025 · Paper 1, Chemistry question 2

Principle. The absorbed wavelength is inversely related to the crystal-field splitting: a larger $\Delta_o$ means a shorter $\lambda_{max}$.

Ligand strength. By the spectrochemical series, $\ce{CN-}>\ce{NH3}>\ce{H2O}>\ce{Cl-}$. Replacing one $\ce{NH3}$ by $\ce{H2O}$, or by the weaker $\ce{Cl-}$, lowers $\Delta_o$. So $\Delta_o$ falls in this order:
$$\ce{[Co(CN)6]^3-}>\ce{[Co(NH3)6]^3+}>\ce{[Co(NH3)5(H2O)]^3+}>\ce{[Co(NH3)5Cl]^2+}.$$

Wavelength. $\lambda_{max}$ increases in the same sequence, which is option (A).