Which of the following compounds show colour due to d-d transition?
- K2Cr2O7
- CuSO4 5H2O
- KMnO4
- K2CrO4
Answer
(2) CuSO4 5H2O
Solution
A d–d transition needs a partly filled d subshell (d1 to d9).
(A) K2Cr2O7: Cr(+6), 3d0 ⇒ (A) is coloured by charge transfer
(B) CuSO4 5H2O: Cu2+, 3d9 (one unpaired electron) ⇒ (B) is blue due to d–d transition
(C) KMnO4: Mn(+7), 3d0 ⇒ (C) is coloured by charge transfer
(D) K2CrO4: Cr(+6), 3d0 ⇒ (D) is coloured by charge transfer
∵ d0 ions have no d electron to excite, only Cu2+ shows d–d colour.
Answer: (2)