Question

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

• A. $KMn{O}_4$
• B. $K_{2}Mn{O}_4$
• C. $MnS{O}_4$
• D. $MnC{l}_2$

Answer 1

The correct option is A $KMn{O}_4$

$KMn{O}_4$ 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 $Mn-O$ 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.