In Al2Cl6, the covalency of aluminium is

→ Sizes of Al and Cl are large, therefore in between the Al and Cl "π" bond formation is not possible.→In order to get octet configuration around the "Al" AlCl3 undergoes dimerisation and forms Al2Cl6  dimer AlCl3 achieves stability by forming a dimer Al2Cl6.In the dimer around the Al four fonds are present and the covalency of Al in Al2Cl6 is four.