in KMnO4 and K2Cr2O7, Mn7+ and Cr6+ with no unpaired electron in the d-orbital still the compounds are coloured. why?
Colour of KMnO4 and K2Cr2O7 is due to ligand to metal charge transfer. Charge transfer takes place from the filled orbital of the ligand to the vacant orbital of the metal atom. This leads to absorption of light of certain wavelength and we see color.