Coordination Number of Aluminium in Crystalline Aluminium Chloride

In the solid (crystalline) state, anhydrous aluminium chloride ($\text{AlCl}_3$) adopts a layered ionic lattice structure (isostructural with $\text{YCl}_3$ or closely related to the $\text{CdCl}_2$ type structure).

In this crystal lattice:

1. The chloride ions ($\text{Cl}^-$) form a cubic close-packed (ccp) array.
2. The aluminium ions ($\text{Al}^{3+}$) occupy two-thirds of the octahedral interstitial voids between alternating layers of chloride ions.
3. Consequently, each $\text{Al}^{3+}$ ion is octahedrally coordinated by six $\text{Cl}^-$ ions.

(Note: Although $\text{AlCl}_3$ exists as a dimeric molecule, $\text{Al}_2\text{Cl}_6$, with a coordination number of 4 in non-polar solvents and in the vapor phase at moderate temperatures, in the crystalline state, it forms an extended octahedral coordination network).

Therefore, the coordination number of $\text{Al}$ in crystalline $\text{AlCl}_3$ is 6.