[Darwin]

Hey

To me it seems wrong that MnO4- has an intense purple colour since it should have zero d-electrons. So can anyone tell me, why this is in fact the case?

[Wald_ron]

The color in KMnO4 arises from an electronic transition, but it is actually 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, in which photons promote an electron from the highest energy molecular orbital in one of the Mn-O bonds 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.

MnO4- : The permanganate ion having tetrahedral geometry is intensely purple due to strong absorption involving charge transfer from MO derived primarily from filled oxygen p orbitals to empty MO derived from manganese(VII).

[Darwin]

Thanks. That was just what I needed!