Explain in detail- why Al2Cl6 is obtained as a dimer but BCl3 as a mon.docx

Explain in detail, why Al2Cl6 is obtained as a dimer but BCl3 as a monomer with special reference to the boning involved in these systems?
Solution
BCl3 and AlCl3 both are electron deficient molecules because there are six electrons present in the valency shell of central atom. There is a strong tendency for both the compounds to gain two electrons and complete their octet. For both the molecules, the central atom has sp2 hybridisation and trigonal planar geometry. The sp2 hybridisation has one s and two p orbitals, and one unhybridised p orbital.
The unhybridised p orbital is perpendicular to the trigonal plane of the molecule. Since it is empty, it can receive two electrons from a donor. Also, Chlorine atoms have three lone pairs.
The deficiency of electrons of Boron in BCl3 is overcome by formation of co-ordinate bond between the lone pair of Chlorine atom and the empty unhybridised p orbital of Boron atom. The bond formed is forming p?–p? and no dimer is formed.
In AlCl3, the electron deficiency of Aluminium atom is overcome through the formation of co-ordinate bond between lone pair of Chlorine atom of another AlCl3 molecule and the empty unhybridised p orbital of Aluminium atom. Hence, dimer is formed.
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1. Explain in detail, why Al2Cl6 is obtained as a dimer but BCl3 as a monomer with special
reference to the boning involved in these systems?
Solution
BCl3 and AlCl3 both are electron deficient molecules because there are six electrons present in
the valency shell of central atom. There is a strong tendency for both the compounds to gain two
electrons and complete their octet. For both the molecules, the central atom has sp2 hybridisation
and trigonal planar geometry. The sp2 hybridisation has one s and two p orbitals, and one
unhybridised p orbital.
The unhybridised p orbital is perpendicular to the trigonal plane of the molecule. Since it is
empty, it can receive two electrons from a donor. Also, Chlorine atoms have three lone pairs.
The deficiency of electrons of Boron in BCl3 is overcome by formation of co-ordinate bond
between the lone pair of Chlorine atom and the empty unhybridised p orbital of Boron atom. The
bond formed is forming p?–p? and no dimer is formed.
In AlCl3, the electron deficiency of Aluminium atom is overcome through the formation of co-
ordinate bond between lone pair of Chlorine atom of another AlCl3 molecule and the empty
unhybridised p orbital of Aluminium atom. Hence, dimer is formed.