Question

Can you explain up pi-d pi bonding inin PO4 3-??

Answer 1

When a bond is formed between two atoms where one atom is having one vacant orbital and another is having one lone pair of electrons, then this electron pair can be donated to that respective vacant orbital and the bonding is called p(pi) - p(pi) or p(pi) - d(pi). It depends on the orbital to which the electron pair is donated and from which the electron pair is donated. 
In the case of phosphate ion, bonding is due to p(pi) - d(pi) bonding. The vacant 3d-orbitals of P overlap with the filled 2p orbitals of O.

Answer 2

Therefore d-pi- p-pi refers to when one d orbital and p ... so 1 s and 3 p orbitals will used for making sigma bonds .