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Question

Which of the following compounds has colour but no unpaired electrons?

A
KMnO4
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B
K2MnO4
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C
MnSO4
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D
MnCl2
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Solution

The correct option is A KMnO4
KMnO4 is colored due to an electronic transition, which is not a d-d transition, since the Mn in this compound has no d-electrons. It arises from a charge transfer reaction within the molecule, where an electron from the highest energy molecular orbital in one of the MnO bonds is transferred to an empty d orbital on the manganese. This promotion is equivalent to the energy of a yellow photon, so yellow light is absorbed leaving us to see purple, the complementary color.
Hence, it is an example of Ligand to Metal Charge Transfer.

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