Silane
Silane is an inorganic compound with the chemical formula SiH₄, representing the simplest hydride of silicon and serving as the silicon analogue of methane. Silane was first isolated in 1857 by the German chemists Friedrich Wöhler and Heinrich Buff, who obtained it from the reaction of aluminum silicide with hydrochloric acid.[1] It is a colorless, pyrophoric gas that ignites spontaneously upon contact with air, exhibiting a sharp, repulsive odor and high toxicity by inhalation.[2] With a tetrahedral molecular geometry, the central silicon atom is bonded to four hydrogen atoms at bond angles of approximately 109.5°, and a Si-H bond length of 1.4798 Å.[3] Silane is produced industrially through methods such as the reaction of magnesium silicide (Mg₂Si) with hydrochloric acid (HCl), or by reducing silicon tetrachloride (SiCl₄) with hydrogen over a hot silicon wire.[1] These processes yield the gas, which must be handled under inert conditions due to its reactivity.[2] Chemically, silane decomposes slowly in water to form silicates and hydrogen gas, and it reacts vigorously with oxidizing agents.[2] The compound's primary applications lie in the electronics industry, where it serves as a precursor for chemical vapor deposition (CVD) to produce polycrystalline and amorphous silicon films used in semiconductors, solar cells, and photovoltaic devices.[1] Additionally, silane acts as a doping agent in solid-state devices and contributes to the synthesis of silicon nitride and carbide layers.[2] Due to its extreme flammability and toxicity—classified with an LC50 of 9,600 ppm in rats—strict safety protocols, including ventilation and protective equipment, are essential in its handling.[2]Structure and Properties
Molecular Structure
Silane has the chemical formula SiH₄ and serves as the silicon analog of methane (CH₄), representing the simplest member of the silicon hydride family.[2][4] The molecule adopts a tetrahedral geometry around the central silicon atom, with all four Si-H bonds equivalent and H-Si-H bond angles of approximately 109.5°. The Si-H bond length is 1.48 Å.[5] The Si-H bonds exhibit slight polarity due to the electronegativity difference between silicon (1.90) and hydrogen (2.20), resulting in a partial positive charge on the silicon atom and the reverse polarity compared to C-H bonds in methane.[4][6] The systematic and accepted IUPAC name for SiH₄ is silane, with the trivial name monosilane also in common use.[7][8] Isotopologues such as deuterated silane (SiD₄) are employed in spectroscopic studies to facilitate analysis of vibrational and rotational spectra; SiD₄ can be prepared via catalytic hydrogen-deuterium exchange reactions on silane or by analogous reduction methods using deuterated reagents.[9][10]Physical Properties
Silane is a colorless gas that is odorless in pure form but may exhibit a repulsive odor due to impurities at room temperature and atmospheric pressure. Its molecular weight of 32.117 g/mol contributes to its gaseous state under standard conditions.[11][12] Key thermodynamic properties of silane include a melting point of -185.4 °C and a boiling point of -111.9 °C, indicating it liquefies only at very low temperatures. The critical temperature is approximately -3 °C, above which silane cannot be liquefied regardless of pressure. At standard temperature and pressure (STP, 0 °C and 1 atm), its density is 1.44 g/L, roughly twice that of air, which affects its behavior in mixtures and containment systems.[2][11][13]| Property | Value | Conditions |
|---|---|---|
| Melting point | -185.4 °C | 1 atm |
| Boiling point | -111.9 °C | 1 atm |
| Critical temperature | -3 °C | - |
| Density | 1.44 g/L | STP (0 °C, 1 atm) |