The quest for triplet ground state silylenes

The scarcity of triplet silylenes compared with triplet carbenes can be understood in terms of the sizes of the valence orbitals. The larger size of the silicon orbitals leads to a decrease in the repulsion of the nonbonding electrons in the singlet state and hence their energy-lowering separation in the triplet state is less capable of compensating an attendant promotion energy. Calculations suggested that the effect of bulky substituents must be supplemented by a reduction of their electronegativity in order to reduce the promotion energy to the point that a triplet ground state can be achieved at an attainable bond angle. The culmination of this approach has been the generation of a silylene (t-Bu)3SiSiSi(iPr)3 that appears to react from its triplet ground state.