Aluminium chloride exists as dimer, ${\mathrm{Al}}_2{\mathrm{Cl}}_6$ in solid state as well as in solution of non-polar solvents such as Benzene . When dissolved in water it gives

When the dimer of Aluminium  chloride dissolved in water form Hexa-aqua Aluminium (III) chloride complex it is divided into three chloride ions and one complex ion

${\mathrm{Al}}_2{\mathrm{Cl}}_6+12{\mathrm{H}}_2\mathrm{O}\to 2\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{Cl}}_3$

$\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{Cl}}_3\to 2{\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}+3{\mathrm{Cl}}^{-}$