Please do not adjust margins strain energy, which is the energy required to deform the reactants, and the interaction energy between the deformed reactants. The energy decomposition analysis further decomposes the interaction energy into three physically meaningful energy terms: the classical electrostatic interaction, the destabilizing steric Pauli repulsion, which arises from repulsion between closed-shell orbitals, and the orbital interactions, which account for charge transfer (HOMO-LUMO interactions) and polarization effects. By applying the ASM and EDA, the study shows that the H2 activation barrier increases when going down the Group 14 elements due to a worsening of the backward donation from the filled lone-pair orbital of the metallylene to the s*-orbital of H2, Scheme 2b. The filled lone-pair orbital goes down in energy and becomes more diffuse, Figure , which results in a larger, less favorable HOMOE-LUMOH2 gap and a reduced orbital overlap. The stabilization of the HOMOE is traced back to a reduced antibonding character that originates from the smaller orbital overlap between the ns orbital of the central tetrel element and the in-phase ligand orbitals as shown in Figure and is explained in detail in Ref 31. Another very similar explanation for the decrease in HOMO energy from C to Si to Ge to Sn is the lower capability to form hybrid orbitals. As the atomic number increases down Group 14, relativistic effects will be more pronounced, causing the valence s orbital to be stronger contracted than the valence p orbitals, i.e. a size disparity between s and p orbitals becomes significant in the heavier elements, unlike the nearly equal 2s and 2p orbitals in carbon. As a result, heavier elements favor the ns²np² configuration over sp hybridization and have a lower HOMO energy, thereby becoming less nucleophilic.