Question

Why pi-donor ligand interact with metal to get low delta o, but pi-acceptor ligand has high delta o

Answer 1

pi acceptor: metal to ligand pi bonding. Here the ligand has empty orbitals that can engage in metal to ligand pi bonding. The lowest unoccupied molecular orbitals(LUMO) of the ligand are the pi orbitals. The energy of bonding here is greater than the corresponding anti-bonding sigma orbitals and the pi orbitals are filled by metal d orbitals.Hence the strength of the bond is higher. Hence a larger delta. Electrons from metal d-orbitals occupy pi orbitals contributed by the ligands.

pi donor: ligand to metal pi bonding. pi electrons from the metals are donated to the metal ion. Here the pi orbitals are filled but the energy is not higher than the anti bonding orbital from sigma bonding. It is filled by the electrons from the metal d orbitals. Being the Highest occupied molecular orbital(HOMO), energy decreases. Hence the smaller delta.