Why do transition metals form coloured compounds?
Transition elements have 3d orbitals with the same energy level however when molecules/ligands form dative covalent bonds with a metal ion, the electron repulsion causes the 3d orbitals to split into lower and higher energy levels.The energy required to cause an electron to jump from a lower 3d orbital to higher 3d orbital corresponds to a certain wavelength of visible light. This wavelength is absorbed while the other wavelengths of light pass through which gives the compound its colour. The complement of the colour absorbed is seen.
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