Why do transition metal ions form coloured complexes?
1 Answer
It's been a little while since I've taken inorganic but I'll give it a shot. Consider the fact that transition metals have d-electron shells that are partially filled. Transition metals commonly form coordination complexes which are molecules that contain a central transition element surrounded by ligands. These structures can form a variety of different geometries depending on the d-electron count of the molecule.
Different geometries of coordination complexes have different energy differences between the d-electron orbitals. This is the basis of what crystal field theory is about. The variety of differences in energy across different molecules causes light to be absorbed at different energies/wavelengths forming all kinds of neat colors.
