Question:
Describe the change in hybridisation (if any) of the Al atom in the following reaction.
$\mathrm{AlCl}_{3}+\mathrm{Cl}^{-} \longrightarrow \mathrm{AlCl}_{4}^{-}$
Solution:
The valence orbital picture of aluminium in the ground state can be represented as:
The orbital picture of aluminium in the excited state can be represented as:
Hence, it undergoes sp2 hybridization to give a trigonal planar arrangement (in AlCl3).
To form AlCl4–, the empty 3pz orbital also gets involved and the hybridization changes from sp2 to sp3. As a result, the shape gets changed to tetrahedral.