Phosphorus: sp3 hybridized Fluorine (each one of three): sp3 hybridized Back bonding can occur between only PURE orbitals ie those which are unhyridized. Let's look at e- config. of both of them to be a little clear of what I'm asking: P: F: Since fluorine is sp3 hybridized, the orbitals ie either of px, py or pz (2 of which are filled and are lone pairs on F of the 3 total lone pairs) can't participate in back bonding with the unhiybridized d orbital of Phosphorus. My question is, then how does it show back bonding? I know it does, but how?