Question

Boron trichloride exist an Omar why aluminium trichloride exist as dimer why

Answer 1

The electron deficiency (octet) of Boron in BCl3 is compensated (completed) by formation of co-ordinate bond between the lone pair of peripheral Chlorine atom and the empty unhybridised p orbital of Boron atom, forming pπ–pπ bonding. Thus, the electron deficiency of BCl3 is compensated by formation of pπ–pπ bonds within the molecule so nodimer forms.

However, the electron deficiency of Aluminium atom in AlCl3 is compensated (completed) by formation of co-ordinate bond between lone pair of Chlorine atom of another AlCl3 molecule and the empty unhybridised p orbital of Aluminium atom, thus forming dimer.