D-orbitals play a crucial role in molecular hybridization by expanding an atom’s valence shell to accommodate more electrons for bonding.

Molecular hybridization is a fundamental concept in chemistry that describes how chemical bonds are formed in molecules. This process involves the mixing of atomic orbitals to create new hybrid orbitals, which can hold more electrons and facilitate the formation of additional bonds. D-orbitals are particularly significant in this context, especially for elements found in the third period and beyond of the periodic table.

These elements possess d-orbitals within their valence shell, which can participate in bonding. As we move down the groups in the periodic table, the energy differences among the s, p, and d orbitals diminish. This allows the d-orbitals to become involved in hybridization, a process known as d-orbital hybridization. As a result, these elements can form more bonds than typically expected. For instance, phosphorus in $PCl_5$ and sulfur in $SF_6$ illustrate this phenomenon.

D-orbitals are especially important in transition metals, which feature partially filled d-orbitals. These d-orbitals can hybridize with s and p orbitals to create hybrid orbitals capable of forming bonds. This capability accounts for the high coordination numbers often observed in transition metals, as they can establish numerous bonds due to the availability of d-orbitals for hybridization.

In summary, d-orbitals significantly enhance molecular hybridization by expanding the valence shell of an atom, allowing it to accommodate more electrons for bonding. This is particularly relevant for elements in the third period and beyond, as well as for transition metals, where d-orbitals hybridize with s and p orbitals to facilitate the formation of additional bonds.