The interaction of molecular hydrogen with some d6-ML5 and d8-ML4 metal complexes is studied by means of Extended Huckel calculations. In a dissymétric ligand field (σ-donor and π-acceptor ligands), the structure of the complex is governed, for the d6-ML5 metal fragment, by the amplitude of electron transfer toward [math], which is maximum for H2 lying in the plane of σ-donors. On the other hand, the structure of the complex with the d8-ML4 metal fragment is governed by the four-electron repulsion between z2 and σH2 orbitals : the addition of H2 leading to the dihydride adduct is easier in the plane of the π-acceptor ligands which decrease this repulsion at the transition state
