Substituent increments for the activation and reaction energy. With the energy profiles of the H2 activation for 598 silylenes at hand, we became interested in deducing systematic patterns between the substituents and the resulting energies, that would allow for a facile prediction of the activation potential of new silylenes. Based on the Benson group theory in thermochemistry, we hypothesized that the calculated reaction energies and activation barriers for the hydrogen activation can be expressed as a sum of contributions from each substituent bound at the silicon center. This procedure was chosen in analogy to the "Bag of Substituents" first used by Tolman and recently by Gensch et al. to describe the properties of phosphine ligands in transition metal catalysis based on the sum of their substituent contributions. To verify our hypothesis, we described all silylenes as a matrix of all unique substituents, with each matrix element expressing how often (0, 1, 2) a substituent is present in the silylene. Multivariate linear regression (Ridge) with a train-test-split of 70:30 led to a highly predictive model (Figure ), which confirmed the additivity of the substituent energies increments as reflected in the high quality coefficients of determination. For example, in case of the activation energy a mean absolute error (MAE) of only 9.1 kJ/mol for the test set (R 2 test = 0.95) was determined. Additionally, we analyzed, how often a substituent must be present in the training data to obtain good results from the fit. Obviously, each substituent must be represented at least once in the training data, but slightly better results are obtained if each ligand is represented at least twice (for details see SI, Figure ). Thus, models with larger proportion of the test set in the train-test-split still perform well. The prediction quality of the model for the reaction energy ΔGH2 was likewise very good (Figure ), and sufficiently accurate, so that we concluded that ΔG ‡ and ΔGH2 of the H2 activation can be expressed simply by the sum of their substituent increments. The final increments for all 40 substituents were obtained from the fit by the substituent coefficient and intercept. The bar plot of increments (Figure ) colored by the elements in αposition to the silicon center shows a strong dependence between the increment and this element. All substituents within each group have similar increments thus allowing for a rough estimation of the impact of a specific substituent on the activity of silylenes towards hydrogen. For example, boryl and silyl groups in general lead to low activation barriers -in line with the silylenes hitherto known to activate H2 (Figure1) -but highly stable products from which H2 elimination or transfer will be challenging to realize. On the other hand, N-and O-substituents lead to the reverse trend, i.e. high activation barriers and less exergonic reaction profiles, suggesting that a combination of these substituents with boryl or silyl groups might be better suited for realizing catalytic turn-over at mild conditions. Interestingly, phosphanyl substituents -such as those studied experimentally by Izod and coworkers for germylenes -occupy an intermediate position suggesting a so far untapped potential of these silylenes in small molecule activation. Of all the substituents studied, they most likely would lead to a possible H2 transfer judged based on the required energy for the back reaction. Nonetheless, a truly reversibly hydrogenation reaction will not be possible with these (or any other) substituents. However, this does of course not exclude hydrogenation reactions of unsaturated compounds such as alkenes, carbonyl compounds or imines, for which hydrogen transfer has a different activation barrier than the dehydrogenation reaction studied here.