• # question_answer 91)   $BC{{l}_{3}}$exists as monomer whereas $AlC{{l}_{3}}$ is dimerised through halogen bridging. Give reason. Explain the structure of the dimer of $AlC{{l}_{3}}$ also.

$BC{{l}_{3}}$ exists as monomer in spite of the fact that it is an electron deficient compound. It achieves stability by forming $p\pi -p\pi$ back bonding. Chlorine transfers two electrons to vacant 2p-orbital of boron. All the three bond lengths are same. Thus, the back bonding giving double bond character is delocalized in the molecule. $AlC{{l}_{3}}$is also an electron deficient compound. Since, aluminium has larger size than boron, the back bonding is not possible in$AlC{{l}_{3}}$. Aluminium metal complete its octet by forming coordinate bond with chlorine atom of the other molecule,$AlC{{l}_{3}}$. Thus, coordinate bond forming bridges by chlorine atoms between two Al atoms make a dimer molecule.