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.