Which transition metal is known for forming colored compounds?

Iron is known for forming colored compounds due to the presence of partially filled d orbitals, which is a characteristic feature of transition metals. When light interacts with these d orbitals, the electrons can absorb specific wavelengths of light, leading to the transmission of complementary colors. This phenomenon results in various colored compounds when iron is involved in bonding with different ligands or in different oxidation states.

In contrast, aluminum, sodium, and calcium are not transition metals. Aluminum is a post-transition metal and typically forms colorless compounds. Sodium and calcium are alkali and alkaline earth metals, respectively, and also tend to form colorless ionic compounds. These elements lack the d orbitals necessary for the formation of colored complexes, which is why they do not typically produce colored compounds like iron does.