Anhydrous `AlCl_(3)` is covalent but `AlCl_(3). 6H_(2)O` is ionic in nature. How would you account for this?

Explanation

Anhydrous `AlCl_(3)` is covalent while `AlCl_(3).6H_(2)O` is ionic in nature. The third ionisation energy of aluminium is very high. Therefore, a large amount of energy is needed to convert Al atom into `Al^(3+)` ion.
`Al(g)+"5114 kJ mol"^(-1)rarrAl^(3+)(g)+3e^(-)`
Therefore, aluminium prefers to form covalent bonds with chlorine atoms forming covalent `AlCl_(3)`. However, when dissolved in water, it undergoes hydration liberating hydration energy.
`AlCl_(3)+6H_(2)Orarr underset([Al(H_(2)O)_(6)]^(3+)+3Cl^(-))underset("Hydrated")(AlCl_(3).6H_(2)O)+"Energy"`
The hydration energy released provides energy for the removal of 3 electrons from aluminium atom. In fact, aluminium does not exist as simple aluminium ion `(Al^(3+))` but in the form of a complex ion `[Al(H_(2)O)_(6)]^(3+)`. Thus, hydrated aluminium chloride is ionic in nature.