CBSE Class 12-science Answered

Back-bonding  is  the  special  type  of  bond which  is  formed between sp²   hybridised  central atom which must  have empty  p-orbital .  Due  to  this  empty p-orbital it  pulls the  electron  from the  another  p-orbital which is  near by  it .  This  type  of  bond  formation is  known  as  back bonding  or  p?  -p? bond.

 

Due  to  the  presence  of  empty  p-orbital molecule  have  tendency  to  attaract the pair of  electron & the  species which  attract  the  pair  of  electron is  termed  as  lewis  acid .  among  BF₃   BCL₃   BBR₃   BI₃  this  type  of  bonding  is  found .  all  these  are  electron deficient  molecules. So  they  must  be  acidic  but  in  BF₃  the  electron deficiency  of  boron  is  vanished  hence  electron  attracting  tendency  of  boron  will  be  least. This makes BF₃   a  weak  acid.

 

 

Similarly, d ?-p ? is also an example of back bonding. This is commonly seen in metallic compounds.

Both ? bonds in XeO₂F₂ are of type is d ? (OF XENON) p ? (of oxygen).The reason is hybridisation of Xe in compound is sp³d . so no chance of p ? p ? bonds(all 3 p orbitals of Xe are used in hybridisation) it has 2 d ? p ? bonds.