Theoretical study of the metal-metal interaction in dipalladium(I) complexes

Correlated ab initio calculations have been performed on three dipalladium(I) complexes. These compounds differ both by the metal-metal interaction and by the metal-ligand interaction. The [Pd2Cl2(mu-H2PCH2PH2)(2)] complex exhibits a sigma overlap between the two binding metallic orbitals and has no bridging ligand. In [Pd2Cl4(mu-CO)(2)](2-), the leading interaction between the two palladium involves a pi overlap between the metallic orbitals and goes through the two bridging CO ligands. In [Pd2Cl2(mu-CO)(mu-H2PCH2 PH2)(2)], a single CO ligand bridges the two palladium atoms which interact through a hybrid sigma-delta overlap. The three compounds also differ by the metal-metal distances. Surprisingly enough, while the palladium atoms are formally d(9) in all these complexes, none of them is paramagnetic. We propose here a detailed analysis of the electronic structures of these compounds and rationalize their chemical structures as well as the role of back-donation in the CO bridged compounds. Finally, since highly correlated treatments are used to describe these complexes, a detailed study of the role of both non-dynamical and dynamical correlations is performed. Concerning the [Pd2Cl4(mu-CO)(2)](2-) complex, this analysis has revealed that the complex is not bound at the lowest correlated levels of calculation and therefore dynamical correlation is alone responsible for its binding energy.