CBSE Class 12-science Answered

Co-ordination compounds exhibits color, which is attributed to the crystal field theory, corresponding to the d-d transition of elements. Say for example, excitation of an electron from the empty state say for example e_.g., will absorb light that will excite the electron from t_2g to e_g as in case of [Ti(H₂O)₆]³⁺. When light corresponding to the energy of the yellow-green region is absorbed by the complex, this would excite the electron from t_2g level to the e_g level (t_2g¹eg⁰-> t_2g⁰e_g¹). Thus the complex is violet in color.

But in the absence of ligand there is no crystal field splitting and substance become colourless as in case of [Ti(H₂O)₆]Cl₃, when water molecules are removed it becomes colourless.