why kmno4 and k2cr2o7 are coloured.

Potssium dichromate(K2Cr2O7) and potassium permangnate(KMnO4) both are coloured compounds.The purple colour KMnO4 and reddish yellow colour of K2Cr2O7 is due the charge transfer spectra.The deep purple color of the solution of KMnO4 is because-- an electron from a “oxygen lone pair” character orbital is transferred to a low lying Mn orbital and dichromate ion Cr₂O₇^2-- gives crystals of potassium dichromate reddish yellow colour.In both of compounds there is anioin-to-cation charge transfer which is also reffred to as ligand to metal charge transfer.In KMnO4 transfer occurs from oxygen anion to Mn cation and in K2Cr2O7 it is form oxygen anion to chromium cation.

The co-ordinate covalent is there between Mn+7 and o-2 in Mno4- complex compound! In these compound oxygen has extra lone pairs to donate to manganese metal so there is actually charge tranferred spectra and not d-d transition because for d-d transition there has to be electrons in Mn+7 metal ion! Thank you

Color of KMnO4 arises from electronic transition but it is not actually d-d transition as Mn in this compound has no d electrons this promotion is equivalent to energy of yellow photon so yellow light is absorbed leaving us to see purple !!!