Question

Boron halides are monomeric while AlCl₃ forms dimer. Why?

Answer 1

==>In AlCl3,

Al has 6 valence e− now to complete its octet it needs 2 more e−. So it accepts a pair of electron from the Cl of the neighboring AlCl3. This donation of electron results in the formation of dimer.

==>But in BCl3,

due to the small size and the extent of π-bonding in B results in the short bond length, which also prevents Cl to form coordinate bond. And due to this short bond length when they dimerise chlorine atoms are closer than that in AlCl3 and it breaks so it exist as monomer.

Hope you understand.

Thank you :)