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.

Answer:

  \[BC{{l}_{3}}\] 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.