The reason for the colour of potassium dichromate (K2Cr2O7) is ligand to metal charge transfer (LMCT) transition. In Cr2O72-, Cr is in +6 oxidation state and have all the 3d-orbitals vacant. Each Cr6+ ion is surrounded tetrahedrally by four oxide ions. All oxide ions have filled 2p-orbitals. There is a transfer of an electron from the filled 2p-orbitals of O2- to the vacant 3d-orbitals of Cr6+ ion. Since 2p-orbitals of the O2- is ungerade and the 3d-orbitals of Cr6+ are gerade, therefore, electron transition from 2p-orbitals of the O2- to 3d-orbitals of Cr6+ ion is Laporte allowed and there is also no change in spin multiplicity during electronic transition. Therefore, transfer of an electron is Laporte and spin allowed. Therefore, potassium dichromate, K2Cr2O7 (or Cr2O72- ion) is coloured.
Answer: The dichromate (Cr2O72-) ion is orange in colour.
Answer: Chromate (CrO42-) is yellow in colour because of ligand to metal charge transfer (LMCT) transition. There is a transfer of electron from the 2p-orbital of O2- (ligand) to 3d-orbital of Cr6+ ion (metal).
Why is [Ti(H2O)6)] 3+ violet in colour. The violet/purple colour of [Ti(H2O)6)] 3+ ion is due to absorption of blue-green light and transition of electron of the t2g orbitals to one of the degenerate eg ortibals. The absorption spectrum of [Ti(H2O)6)] 3+ reveals that the d-d transition occurs with a single broad peak.
What is the difference between d-d transition and charge transfer transition. How do we distinguish between d-d and MLCT transistions.