In the gaseous phase, aluminium chloride at low temperatures (420 – 480 K) exists as

Aluminum chloride is a dimer in the gas phase (Molecular formula of ${\mathrm{Al}}_2{\mathrm{Cl}}_6$).

The aluminium atoms combine with the three Cl atoms to produce three sp2 hybridised bonds in each unit of the complex, or ${\mathrm{AlCl}}_3$. The ${\mathrm{AlCl}}_3$species, however, lack electrons. They must dimerize and produce ${\mathrm{Al}}_2{\mathrm{Cl}}_6$in order to complete their octet as follows: