Definition of the chemical space region and data generation. To explore the chemical space of heavier carbenes applicable in hydrogen activation, we conducted a virtual screening of compounds of type RER' with E = Si and Ge. We decided to focus our studies on silylenes and germylenes which we expected to be the most promising candidates for reversible H2 activation due to the higher stability of the +2 oxidation state compared to carbenes combined with the higher activity and R-E bond strengths relative to the tin and lead analogues. Initially, we focused on silylenes, which we generated in silico by combinatorial enumeration of different R substituents bound to the silicon center. To generate a large but also meaningful chemical space of silylenes, we initially chose substituents previously described in literature to form stable acyclic silylenes or germylenes, some of which were capable of activating dihydrogen (Figure ), and enlarged the library by derivatives of these substituents. This selection was made to ensure synthetic accessibility of the calculated systems. Overall, 40 substituents of different sizes and with different donor atoms (Figure ) were used, including amido, alkyl and aryl, oxylato, thiolato, phosphanyl, silyl, N-heterocyclic iminyl (NHI) and boryl groups. Next, the silylenes were virtually generated from combinations of these substituents and thus used in further studies.