The decrease in reactivity of metallylenes when going down Group 14 has been computationally studied more frequently. Wu et al. examined the reactivity of boryl(phospino)-based metallylenes with water and ethylene by DFT and found higher reaction barriers for the metallylenes with heavier Group 14 atoms and a lower HOMO energy. The lowering of the HOMO was attributed to the smaller capability of larger elements to form hybrid orbitals. Besides that, the authors correlate the reaction barrier with the singlet-triplet gap: a transition state (TS) has greater stability when the singlet-triplet gap is small and thus a lower barrier as previously studied by configuration mixing. From the valence bond theory point of view, the bonding in an activated metallylene (the product) can be described as a combination between the triplet metallylene and the two doublet radicals, for instance, for water splitting the OH radical and the H atom. Wang and Ma investigated H2 activation by cyclic and acyclic silylenes and germylenes with DFT and ab initio methods and correlated the reaction barriers to the HOMO-LUMO gap. Ess and coworkers utilized absolutely localized molecular orbital (ALMO) analysis and quasiclassical trajectories to gain more insights into H2 activation by metallylenes. The decrease in barrier from a carbene to a silylene or germylene was ascribed to electron repulsion instead of HOMO-LUMO interactions. The geometries of the transition state structures differ between the carbene and silylene or germylene. In the transition states of the carbenes, forward-bonding and back-bonding are largest in the so-called non-least motion geometry, while the transition states of the silylenes and germylenes adopt side-on geometries to avoid steric electron-electron repulsion (Figure ).