What is the role of d-orbitals in transition metals?

D-orbitals in transition metals are primarily responsible for their unique chemical properties, including variable oxidation states and coloured compounds.

Transition metals are elements found in the d-block of the periodic table, which includes groups 3 to 12. These metals are characterised by the presence of d-orbitals in their outermost energy levels. The d-orbitals play a significant role in the chemical behaviour of these elements, particularly in their ability to form various oxidation states and coloured compounds.

The d-orbitals can accommodate up to ten electrons, which allows transition metals to have multiple oxidation states. This is because the energy difference between the 4s and 3d orbitals is relatively small, so electrons can be easily removed from either orbital during oxidation. For example, iron can exist in both +2 and +3 oxidation states, which are due to the removal of electrons from the 4s and 3d orbitals respectively.

Another unique property of transition metals, which is attributed to the presence of d-orbitals, is their ability to form coloured compounds. This is due to the d-d transitions, where an electron is excited from a lower energy d-orbital to a higher energy d-orbital. The energy required for this transition corresponds to the energy of light in the visible spectrum, which results in the absorption of a specific colour of light and the reflection of the complementary colour. For instance, the blue colour of copper(II) sulphate is due to the absorption of red light during the d-d transition.

Furthermore, the presence of d-orbitals in transition metals also allows them to act as good catalysts. This is because the d-orbitals can provide a site for reactant molecules to attach, facilitating the breaking and making of bonds during a chemical reaction. For example, nickel is used as a catalyst in the hydrogenation of alkenes due to the availability of d-orbitals.

In conclusion, the d-orbitals in transition metals play a crucial role in their chemical behaviour. They are responsible for the variable oxidation states, the formation of coloured compounds, and the catalytic properties of these elements.