Question

In which one of the following, d-d transition involves absorption in the ultraviolet region

• A. [Cu(H$_2$O)$_4$]$^{2+}$
• B. [Ti(H$_2$O)$_6$]$^{3+}$
• C. [Co(NH$_3$)$_6$]$^{3+}$
• D. [Co(CN)$_6$]$^{3-}$
• E. [Co(NH$_3$)$_5$Cl]$^{2+}$

Hint:

Stronger ligand → larger $\Delta$ → higher energy → UV absorption.

Answer 1

The Correct Option is D

Concept: d–d transition energy depends on crystal field splitting \(\Delta\). Higher \(\Delta\) → higher energy absorption → shifts toward UV region.

Step 1: Use spectrochemical series. \[ CN^- > NH_3 > H_2O > Cl^- \]

Step 2: Identify strongest ligand.
CN$^-$ is strongest → produces maximum splitting.

Step 3: Compare complexes.

• Weak field → visible region
• Strong field → UV region Thus: \[ [Co(CN)_6]^{3-} \text{ has maximum splitting} \]

Step 4: Conclusion. Absorption shifts to ultraviolet region.

Step 5: Final answer. \[ \boxed{(D)} \]