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Elementary reaction mechanisms constitute a fundamental infrastructure for chemical processes as a whole. However, while these mechanisms are well understood for second-period elements, involving those of the third period and beyond can introduce unorthodox reactivity. Combining crossed molecular beam experiments with electronic structure calculations and molecular dynamics simulations, we provide compelling evidence on an exotic insertion of an unsaturated sigma doublet radical into a silicon-hydrogen bond as observed in the barrierless gas-phase reaction of the D1-ethynyl radical (C₂D) with silane (SiH₄). This pathway, which leads to the D1-silylacetylene (SiH₃CCD) product via atomic hydrogen loss, challenges the prerequisite and fundamental concept that two reactive electrons and an empty orbital are required for the open shell, unsaturated radical reactant to insert into a single bond. 

The reactions of the D1-silylidyne radical (SiD; X 2 Π) with deuterium sulfide (D 2 S; X 1 A 1 ) and hydrogen sulfide (H 2 S; X 1 A 1 ) were conducted utilizing a crossed molecular beams machine under single collision conditions. The experimental work was carried out in conjunction with electronic structure calculations. The elementary reaction commences with a barrierless addition of the D1-silylidyne radical to one of the non-bonding electron pairs of the sulfur atom of hydrogen (deuterium) sulfide followed by possible bond rotation isomerization and multiple atomic hydrogen (deuterium) migrations. Unimolecular decomposition of the reaction intermediates lead eventually to the D1-thiosilaformyl radical (DSiS) (p1) and D2-silanethione (D 2 SiS) (p3) via molecular and atomic deuterium loss channels (SiD–D 2 S system) along with the D1-thiosilaformyl radical (DSiS) (p1) and D1-silanethione (HDSiS) (p3) through molecular and atomic hydrogen ejection (SiD–H 2 S system) via indirect scattering dynamics in barrierless and overall exoergic reactions. Our study provides a look into the complex dynamics of the silicon and sulfur chemistries involving multiple deuterium/hydrogen shifts and tight exit transition states, as well as insight into silicon- and sulfur-containing molecule formation pathways in deep space. Although neither of the non-deuterated species – the thiosilaformyl radical (HSiS) and silanethione (H 2 SiS) – have been observed in the interstellar medium (ISM) thus far, astrochemical models presented here predict relative abundances in the Orion Kleinmann-Low nebula to be sufficiently high enough for detection.