Valence bond approach of metal-ligand bonding in the Dewar-Chatt-Duncanson model

Metal-ligand bonding, as considered in the Dewar-Chaft-Duncanson model, is described via an ab initio Valence Bond (VB) approach and applied to typical Pd-L complexes (L = NH3, PH3, CH2, SiH2). A progressive construction of the VB wave function is followed and leads to a very compact, though accurate, description of metal-ligand bonds. A description with the donation interaction only (ligand-metal) is first constructed and enriched so the back-donation interactions (metal-ligand) are also introduced. This latter VB wave function, although being extremely compact, provides bonding energies in agreement with standard (correlated) methods. A comparison between the two VB levels allows a quantification of adiabatic back-bonding energies and reveals very different trends between the ligands considered. A very faint back-donation in Pd-NH3 is found, which contrasts with a significant effect in Pd-PH3. Back-donation is, however, more important in Pd-XH2 complexes. In Pd-CH2, it is such that it even represents the major source of bonding. For Pd-SiH2, back-donation is slightly weaker than donation. The nature of the interaction in these metal-ligand complexes is revealed by the VB wave function analysis. Results are as well rationalized using the simple molecular orbital picture and compared to previous studies.