Abstract.The protonation of the [Pd(H)2(Cl)(NH3)] and [Pd(H)2(NH3)2] taken as models of anionic and neutral square-planar d8 palladium complexes is investigated through SCF, MP2, MP4, CASSCF and CASPT2 calculations, using various basis sets on the metal and the ligands. It is shown that correlation effects, mainly those associated with the covalent character of the metal hydrogen and metal ligand bonds, are important. The importance of diffuse functions on the ligands, especially for the anionic system, is stressed.