Question

what is p - pie p-pie back bonding

Answer 1

If there is bonding between two atoms where one atom is having one vacant orbital and another is having one lone pair of electrons, then this electron pair is donated to that respective vacant orbital ..then this bonding is called p-p or p-d depending upon the orbital to which the pair is donated and from which the electron pair is donated. if both the orbitals are d, then its called d(pi)-d(pi) bonding. these bonding are also called back-bonding bonds.
Eg.
in BF3, B has incomplete octet so a fully filled p(pi) orbital of F donates a lone pair of e to the vacant orbital of B. so here pp(pi)-p(pi) bonding occurs and is known as back-bonding also.