Anhydrous ${\mathrm{AlCl}}_3$ is covalent but ${\mathrm{AlCl}}_3.6{\mathrm{H}}_2\mathrm{O}$ is ionic because

the hydration energy of ${\mathrm{AlCl}}_3$ is very high

the hydration energy of ${\mathrm{AlCl}}_3$ compensates for the high ionization energy of aluminum

the ionization energy of Al is very low

${\mathrm{AlCl}}_3$has a plane triangular structure