BCl3 exists as monomer whereas AlCl3 is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of AlCl3 also.

Boron halides do not exist as dimer due to small size of boron atom which makes it unable to accommodate four large sized halid ions. AlCl3 exists as dimer. Al makes use of vacant 3p-orbital by coordinate bond i.e., Al atoms complete their octet by forming dimers.