Question

Why is there no Pi-backbonding in BeCl2?

Answer 1

In solid state BeCl2 exists as a polymer having sp3 hybridised. So no back bonding here. But in gaseous state it exists both as dimer and as monomer. The case with dimer is the same. But in monomer it has sp hybridisation. Well in order to complete it's octet it should undergo back bonding.. But back bonding depends on the electronegativity difference. Higher the difference higher is the energy difference in their orbitals. Higher the energy difference lesser is the extent of back bonding. This extent is so less for group 2 elements that back bonding doesn't necessarily exist for group 2 halides.