Aluminium Chloride dimer and Ammonia trifluoroborane

Aluminium is in group 3 of the periodic table. For aluminium chloride AlCl₃, the difference in electronegativity between aluminium and chlorine is not sufficient to form a simple ionic bond.
At room temperature, aluminium chloride forms an ionic lattice with a lot of covalent character.
At 2atm or higher, aluminium chloride melts at 192°C. At lower pressures, it sublimes at about 180°C forming Al₂Cl₆ dimers by dative covalent (coordinate) bonding.

When ammonia (NH₃) is added to boron trifluoride (BF₃) a dative covalent bond (coordinate bond) forms between the nitrogen atom and the boron atom. The nitrogen atom donates its lone pair of electrons to the electron-deficient boron atom.