Based on the properties of the silylenes, compound 4 was predicted to exhibit the lowest barrier to activate dihydrogen. With a required activation barrier of only 106 kJ/mol, this silylene should be capable of activating dihydrogen at or close to room temperature. All other silylenes 1-3 also showed barriers lower than 125 kJ/mol indicating a possible activation at slightly elevated temperatures. Energy prediction of the H2 activation with germylenes. Having established relationships between the substituents in silylenes and their ability to activate H2 as well as a predictive model based on substituent properties, we next became interested in transferring these results to the heavier analogue, the germylenes. Applying a similar procedure for the bag of substituents as described above on a smaller set of germylenes (23 substituents, 198 germylenes) confirmed the additivity of the substituent energy increments for determining the activation barrier and reaction energy (energy values are given in Table ).