24.4 Color and Magnetism
Color of a complex depends on the metal, its oxidation state, and its ligands.
Pale blue [Cu(H2O)6]2+ can be converted into dark blue [Cu(NH3)6]2+ by adding NH3(aq).
A partially filled set of d orbitals is usually required for a complex to be colored.
So, d0 metal ions are usually colorless. Exceptions: MnO4- and CrO42-.