Why boron halides do not exists as a dimer. While `AlCl_(3)` exists as `Al_(2)Cl_(6)` ?

Explanation

Boron atom being small in size is unable to accommodate four large sized halogen atoms (exept F) around it. G However , because of bigger size, Al can easily accommodate four big sized Cl atoms around it. Since in AlCl_(3), there are only six electrons around Al atom, therefore, it completes its octet by accepting a lone pair of electrons from Cl atom of another `AlCl_(3)` molecule. As a result, `AlCl_(3)` exists as a chlorine bridged dimeric structure.