Predictions beyond the data set. Having established an accurate model, we next aimed at evaluating its predictive power. To this end, we investigated the hydrogen activation by the four new silylenes 1-4 (Figure ). We intentionally opted for silylenes with phosphanyl groups, since they have so far little been investigated in silylene chemistry, but revealed to be promising substituents for room temperature H2 activation based on their energy increments (Figure ). We chose substituents, which were not included in the original data set to probe the extrapolation capability of the model, with silylene 4 even consisting of two unknown substituents. To our delight, the predicted barriers were found to match the calculated barriers excellently, with an average deviation of only approx. 3 kJ/mol, thus confirming the predictive power of our model and the possible extrapolation to substituents unseen by the model. The extremely small deviation from the calculated values is impressive, particularly when keeping in mind that only three phosphanyl substituents were included in the original chemical space, showing that this number is sufficient for the model to learn the decisive properties.