alkynes, carbonyls, and imines by H2-activated metallylenes with different tetrel central atoms and ligands (Figure ). In this DFT study, we observed a clear trend among the various metallylenes: a metallylene with a lower H2 activation barrier generally leads to a more stable activated metallylene, which results in a higher hydrogenation barrier. The lowering of the H2 activation barrier from Si to Ge to Sn is caused by a worsening of the back donation from the filled lone-pair orbital of the metallylene to the s*-orbital of H2, as discussed earlier, while the hydrogenation barrier increases as a result of stronger E -H bonds. When going from a nitrogen ligand to a phosphorus ligand, the H2 activation barrier decreases through a difference in hyperconjugation and pyramidalization of the ligand (see before), while the hydrogenation barrier increases. We studied the trends in hydrogenation barrier by the activation strain model and energy decomposition analysis. The decrease in barrier from Si to Ge to Sn is due to a progressively lower energy requirement to break the E-H bonds, as the bond strength decreases from Si-H to Ge-H to Sn-H. The weaker bonds are quantified by a lower bond dissociation energy, Figure . The decrease in reactivity when changing from a nitrogen to a phosphorus ligand stems from a pyramidal PMe2 geometry compared to a trigonal planar NMe2 geometry in their transition state structures. The strong hyperconjugation and thus planar NMe2 geometry results in symmetric orbital shapes, while that is not the case for the PMe2 metallylene, see the HOMO shapes in Figure . The symmetrically shaped HOMO can engage in stronger orbital interactions with H2 because of a higher orbital overlap with the LUMO of H2. The optimal catalyst among the studied metallylenes was identified as one with a low H2 activation barrier and a low barrier for the subsequent hydrogenation reaction. A good candidate has a PMe2 ligand, which significantly reduces the H2 activation barrier, and a large central tetrel atom, such as Sn or Ge, which reduces the hydrogenation barrier. Hydrogenation reactions by H2-activated metallylenes have not yet been performed experimentally to the best of our knowledge. As previously discussed, achieving a thermoneutral H2-activation is critical, and a thermoneutral reaction would inherently exhibit reversibility. Devarajan et al. considered reversibility in their DFT study to identify an optimal catalyst for reversible H2 activation and determined that Ge(BH2)2 was Please do not adjust margins Please do not adjust margins closest to an ideal, with an activation barrier of 6.0 kcal mol -1 and a reaction energy of -23.8 kcal mol -1 . Reversibility has been experimentally observed for a silylene, where the divalent silicon Si(II) inserts readily into its ligand (a Dipp group), forming Si(IV). At elevated temperatures the transformation reverses, regenerating the silylene. A reversible reaction by a phosphinestabilized silylene at room temperature has been observed and the silylene is able to activate Si-H and P-H bonds. Silylene chemistry is further extensively discussed in Reference 84. A reversible photoinduced activation of white phosphorus (P4) has been performed with di-aryl and silyl germylenes and stannylenes, 86 thereby providing an (air-stable) storage compound. Although there is still room for progress in reversible small molecule activation, some follow-up reactions have already been performed. For instance, the hydroboration of ketones and aldehydes (and, more rarely, imines ) has been achieved using germylenes, stannylenes, and even plumbylenes (E = Pb, Scheme 5). Pinacolborane (HBpin) is commonly utilized as a reagent for hydroboration. Metallylenes with different types of ligands have shown activity (Scheme 5a), including those with a phosphine ligand, 80b hydride metallylenes with a bulky amide ligand, N-heterocyclic germylenes and stannylenes, nitrogen or phosphorus coordinated systems via, for instance, a N,C,N pincer ligand, carbonyl-coordinated stannylenes, and other ligands. Various mechanisms have been proposed, primarily differing in which small molecule is first activated, Scheme 5b: 1) the B-H bond of HBpin is activated first by the metallylene, followed by the hydroboration of the ketone/aldehyde, or 2) the carbonyl oxygen of the ketone/aldehyde first coordinates to the metallylene, after which HBpin interacts with the activated ketone/aldehyde, leading to hydroboration. In this mechanism, a ligand (sometimes a single H-atom) plays a role in the first step by coordinating to the carbon of the carbonyl group of HBpin.