We have performed ab initio calculations to determine the degree of σ-basicity and π-acidity of a series of phosphorus ligands in model compounds of the type Pd-PX3, trans-L-Pd-PX3 and (CO)3Pd-PX3 with PX3 = P(CH3)3, PH3, P(OCH3)3, or PF3 and L = CO or NH3. We found that all the phosphine ligands considered are good σ-donors, with PF3 being only a moderately weaker base than P(CH3)3. In this respect, we demonstrate the inadequacy of the proton affinity as a measure of the σ-basicity of a ligand in zerovalent metal complexes. Alkylphosphine ligands are also good π-acceptors, but different from the σ-basicity, the π-acidity increases markedly along the series P(CH3)3 < PH3 < P(OCH3)3 < PF3. We have analyzed the extent to which the phosphorus 3d orbitals are directly involved in the bonding, in particular in the π-back-donation from the metal to the ligand. The P d functions are essential for a correct description of the metal-phosphorus bond. However, their role is that of polarization functions which can combine with the PX3 antibonding orbitals to form hybridized π-acceptor orbitals and not that of orbitals directly involved in the charge transfer from the metal. The π-acidity of P(CH3)3 and PH3 is considerably reduced upon removal of the phosphorus d functions. © 1992, American Chemical Society. All rights reserved.