Aluminium is a Group 13 element located in the third period. Unlike the first member of the group (Boron), which has a maximum covalency of 4 (e.g., [BF₄]⁻), aluminium has vacant d-orbitals in its valence shell, allowing it to expand its coordination number beyond 4 [Source 70]. In an acidified aqueous solution, aluminium chloride dissolves to form the hexaaquaaluminium(III) ion, where the central metal ion is surrounded by six water molecules. The formula of this complex is $[Al(H_2O)_6]^{3+}$. To form this octahedral complex, the Al³⁺ ion (electronic configuration [Ne]) utilizes its empty 3s, 3p, and two 3d orbitals for hybridization, resulting in sp³d² hybridisation [Source 30, 70, 111].