NEET 2025 · ChemistryCrystal fieldPrevious Year Question
The correct order of the wavelength of light absorbed by the following complexes is: A. $\ce{[Co(NH3)6]^3+}$ B. $\ce{[Co(CN)6]^3-}$ C. $\ce{[Cu(H2O)4]^2+}$ D. $\ce{[Ti(H2O)6]^3+}$
- A.C < D < A < B
- B.C < A < D < B
- C.B < D < A < C
- D.B < A < D < C✓
Correct Answer
(D) B < A < D < C
Solution & Explanation
\textbf{Answer:} (D) B < A < D < C \textbf{Solution:} The wavelength of light absorbed is inversely related to the crystal-field splitting $\Delta_o$, since $\lambda = \dfrac{hc}{\Delta_o}$ (larger $\Delta_o \Rightarrow$ shorter $\lambda$). Comparing the field splittings: B $\ce{[Co(CN)6]^3-}$ — strongest field ($\ce{CN-}$, $\ce{Co^3+}$) $\Rightarrow$ largest $\Delta_o$. A $\ce{[Co(NH3)6]^3+}$ — strong field $\ce{NH3}$, next largest. D $\ce{[Ti(H2O)6]^3+}$ — $\ce{H2O}$ with $\ce{Ti^3+}$. C $\ce{[Cu(H2O)4]^2+}$ — weakest field $\Rightarrow$ smallest $\Delta_o$. So $\Delta_o$: B > A > D > C, and therefore the wavelength absorbed increases in the reverse order: B < A < D < C (option D).
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