Transition metals form coloured ions or compounds due to the partially filled d-orbitals. In the presence of solvent molecules (in solutions) or ligands (in complexes) or counter ions (in crystals), the d-orbitals split into two sets. The electrons in transition metal ions which occupy one set of d-orbitals having lower energy can be excited to another set of d-orbitals having high energy by absorbing energy from visible light. Since the energy difference \((\Delta E)\) is small between the two sets of d-orbitals, the light in the visible region only is absorbed by the electron during its excitation. The colour of the transition metal ion is due to the d–d excitation or the d–d transition of the electron. During d–d excitation the electron absorbs one colour in the visible light and thus it appears in the complementary colour of the absorbed light. The complimentary colours can be identified using the Munsell colour wheel (as depicted below).

The number of electrons undergoing d–d transition and the energy difference between the two sets of orbitals decide the colour. The colour of particular transition metal ion, e.g., copper ion which is blue in aqueous solution changes to dark blue in the presence of sufficient ammonia and to green if sufficient chloride ions are added. \(\Delta\)E value depends on the nature of metal ion, the nature of ligands and several other factors. Some metal ions exhibit different colours in different oxidation states.

Which statement is correct about the colour of a transition metal ion?

All are correct

The correct answer is option 4. all are correct.

Let us delve into each statement regarding the color of a transition metal ion:

1. The color is decided by the energy difference between two sets of d-orbitals:

Transition metal ions exhibit color due to the absorption of certain wavelengths of light. This absorption occurs when electrons in the d orbitals of the transition metal ion are excited to higher energy levels.

The energy difference between two sets of d orbitals within the transition metal ion determines which wavelengths of light are absorbed. This energy difference is associated with electronic transitions between these orbitals.

The absorbed wavelengths correspond to specific colors, while the remaining wavelengths are transmitted or reflected, giving rise to the observed color of the transition metal ion.

2. The color is decided by the number of unpaired electrons in d-orbitals:

Unpaired electrons in the d orbitals of a transition metal ion can influence its color. The presence of unpaired electrons introduces additional electronic transitions and affects the intensity of absorption of certain wavelengths of light.

The number of unpaired electrons in the d orbitals contributes to the overall electronic configuration of the transition metal ion and influences the nature of d-d electronic transitions responsible for its color.

3. The color is decided by the absorbed color by the electron for excitation:

When visible light interacts with a transition metal ion, certain wavelengths are absorbed by electrons in the d orbitals, causing them to be excited to higher energy levels.

The absorbed wavelengths of light correspond to specific colors. The color observed in the transition metal ion is a result of the wavelengths that are absorbed by the electrons during excitation.

The color perceived by an observer is complementary to the absorbed wavelengths, as the transmitted or reflected light contains the remaining wavelengths not absorbed by the transition metal ion.

In summary, the color of a transition metal ion is influenced by multiple factors, including the energy difference between d orbitals, the number of unpaired electrons in d orbitals, and the absorbed wavelengths of light during excitation. These factors collectively determine the observed color of the transition metal ion in solution or solid state.