Our group studied the effect of changing the ligand of metallylenes from NMe2 to PMe2 to AsMe2 in H2 activation by DFT. The reaction barrier lowers significantly when changing from a nitrogen ligand to a phosphorus ligand to an arsenic ligand. Analyses, using the ASM and EDA on model systems H3C-E-L (E = C, Si, Ge, Sn, L = NMe2, PMe2, AsMe2), revealed that the difference in pyramidalization of the ligand plays a key role. The NMe2 and PMe2 ligand adopt a trigonal planar geometry in the equilibrium geometry of the metallylene, while the AsMe2 ligand adopts a more pyramidal geometry (Figure ). The ligands in metallylene catalysts adopt a trigonal planar geometry due to a hyperconjugation interaction between the empty np orbital of the central Group 14 element and the occupied np orbital of the nitrogen or phosphorus atom. The nitrogen ligand exhibits a stronger preference for trigonal planarity compared to the phosphorus ligand, which prefers planarity more than the arsenic ligand. This preference is a result of the progressively longer bond from N-E to P-E to As-E bonds. Pyramidalization of NMe2 causes significant steric repulsion between the methyl groups, due to their close proximity, while steric repulsion for PMe2 and AsMe2 remains lower due to the longer P-C and As-C bonds. During H2 activation, the ligands pyramidalize. NMe2 does not undergo Please do not adjust margins Please do not adjust margins substantial pyramidalization, in contrast to PMe2 and AsMe2, Figure . The different preference for pyramidalization has two main effects: i) deformation of the metallylene during the H2 activation requires less energy for AsMe2 as it already adapts a pyramidal shape, whereas pyramidalization of NMe2 or PMe2 costs energy, and ii) the less pyramidal shape and therefore stronger hyperconjugation in NMe2 and PMe2 results in a higher electron density on the metallylene atom that causes a higher steric (Pauli) repulsion with H2, Figure . Thus, the stronger hyperconjugation and less pyramidalization in the metallylene with the nitrogen ligand results in a higher H2 activation barrier compared to one with a phosphorus or arsenic ligand. This explanation is not limited to the H₃C-E-L model systems but also applies to larger, synthetically feasible systems. When comparing a germylene bearing a nitrogen ligand with one bearing a phosphorus ligand synthesized by Aldrige and coworkers, a higher steric (Pauli) repulsion for the metallylene containing the nitrogen ligand causes a higher reaction barrier.