Question

BCl3 exists as a monomer whereas AlCl3 exists as a dimer. Why?

Answer 1

These both are electron deficient molecules as central atom in both of these i.e., B and Al have 6 electrons in valence shell.

So to complete the octet B forms a coordinate bond with Cl atom which has lone pair of electrons and also size difference is not large so there is effective pπ-pπ bonding But in case AlCl3 this coordinate bond is not formed coz of many reasons that is y it exist as dimer.

Hope this helps