Metal - Ligand bonding, as considered in the Dewar-Chatt-Duncanson model, is described via an ab initio Valence Bond (VB) approach, and applied to typical Pd - L complexes (L = NH₃, PH₃, CH₂, SiH₂).

Donation and Back Donation effects are separated using modern Valence Bond (VB) approaches for typical Pd - L complexes.

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 to Metal) is first constructed (black curve), and enriched so the back donation interactions (Metal to Ligand) are also introduced (red curve). This latter VB wave-function, although being extremely compact, provides bonding energies in agreement with standard (correlated) methods. (We note the bond shortening when the back donation are accounted for).

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 simple MO picture, and compared to previous studies.