Silylenes

Silylenes are silicon-centered species featuring a two-coordinate silicon atom with two substituents and a formal Si(II) oxidation state. They are the silicon analogs of carbenes and possess a lone pair on silicon along with a vacant p orbital, giving rise to unusual reactivity and high sensitivity to air and moisture. In contrast to more common silanes, silylenes are divalent and often exist only as transient intermediates, though stabilized variants can be isolated under carefully controlled conditions.

The electronic structure of silylenes can accommodate both singlet and triplet states. The relative energies of these states depend on the substituents attached to silicon, with bulky or π-donor groups capable of altering the state ordering. Singlet silylenes tend to behave as electrophiles and engage in two-electron processes such as cycloadditions and insertions, while triplet silylenes exhibit diradical or radical-like reactivity, including reactions with unsaturated substrates and hydrogen-atom transfer. This state dependence gives silylenes a rich and diverse reactivity profile.

Synthesis and stabilization typically rely on generating the Si(II) center from precursors by photolysis or thermolysis, often at low temperatures, and then stabilizing the species with bulky substituents or through matrix isolation. Common strategies use bulky aryl or amide substituents to provide kinetic protection, allowing the transient species to be studied spectroscopically and leveraged in reactions.

Reactions of silylenes include additions to multiple bonds and insertions into Si–H and C–H bonds, as well as formation of silicon-containing cyclic structures upon interaction with alkenes or alkynes. These transformations contribute to fundamental understanding of silicon valence and offer potential avenues in small-milestone organosilicon synthesis and materials chemistry, though practical use remains limited by stability challenges.