Computational exploration of the reaction energies. For each of the obtained 598 silylenes the energy of the activation barrier R 1 R 2 Si-TS, the activation product R 1 R 2 Si-H2 and the silylene itself R 1 R 2 Si: were calculated (Scheme 1, for detailed information see the Supporting Information). These energies were obtained through an automated procedure depicted in Figure . The initial geometry guesses for the silylenes and the H2 activation products were generated from the corresponding SMILES strings via an initial geometry optimization at the GFN2-xTB level of theory with the GBSA solvent model for THF, followed by a subsequent conformer ensemble generation using CREST at the same level of theory. The lowest energy conformer was then chosen for further optimization at density functional theory (DFT) level (PBE0-D3/df2-svp//PBE0-D3/df2tzvp). For the transition states, the first structure guess was made based on the assumption that R 1 R 2 Si-TS resembles the structure of the silylene R 1 R 2 Si:. Starting from the optimized silylene structure the initial geometry guess for the transition state was obtained by addition of two hydrogen atoms to R 1 R 2 Si: using mean distances, angles and dihedrals obtained from initially calculated transition states. As there are two possible locations for hydrogen atoms at both sides of the silylene, both TS were optimized on DFT level, from which the energetically favored TS was used in the following. Scheme 1. Calculated mechanism for the dihydrogen activation at the silicon atom in silylenes. All energy values are given in Table , S7 and S8 in the Supporting Information. It should be noted that during TS search two different modes of hydrogen activation were observed for some structures: the single-site and cooperative H2 activation across the Si-R linkage. These cooperative H2 activations were mainly found for silylenes with boryl substituents due to the availability of an empty p-orbital at boron, but also for some silyl substituents. Within this work we decided to focus only on single-site H2 activations, and thus tried to obtain only this transition state for each silylene. Transition states which could not clearly be determined as single-site were excluded from the dataset. Furthermore, it must be noted that in some cases the value for the "barrier" (energy difference between R 1 R 2 Si-TS and R 1 R 2 Si:) is negative. This can be explained by the formation of intermediate R 1 R 2 Si-Int (Scheme 1), in which H2 coordinates to the silicon center through an interaction of the σ-bond of H2 and the vacant p-orbital of the silicon center analogous to σ-complexes in transition metal complexes. For highly reactive and instable silylenes, R 1 R 2 Si-Int as well as R 1 R 2 Si-TS are energetically favored compared to the free silylene and H2. In these cases, the energy difference is not equal to the actual barrier of the reaction, but still reflects the impact of the substituent on H2 activation relative to the silylene. Thus, and for clarity, we used the term "barrier" in the following for all energy differences between the free silylene and the H2 activation transition state.