From Diaminosilylenes to Silapyramidanes: Making Sense of the Stability of Divalent Silicon Compounds

Since the discovery of decamethylsilicocene over three decades ago, chemists have successfully isolated a variety of divalent silicon compounds by orchestrating steric and electronic effects to their advantage. Two broad strategies of electronic stabilization appear to have been widely deployed, namely -conjugation as in diaminosilylenes and -complexation as in decamethylsilicocene and silapyramidanes. Herein, we have attempted to identify quantitative metrics for the electronic stabilization of silylenes. Singlet-triplet gaps and electron affinities, both physical observables, appear to be useful in this regard. Thus, the most stable silylenes exhibit unusually high singlet-triplet gaps and very low or negative gas-phase electron affinities. Both metrics signify low electrophilicity, i.e., a low susceptibility to nucleophilic attack. The ionization potential associated with the Si-based lone pair also gives an indication of nucleophilic reactivity.