Silanes

Binary silicon-hydrogen compounds are saturated chemical compounds with the empirical formula SiHn. All contain tetrahedral silicon and terminal hydrides. They only have Si–H and Si–Si single bonds. The bond lengths are 146.0 pm for a Si–H bond and 233 pm for a Si–Si bond. The structures of the silanes are analogues of the alkanes, starting with silane, SiH4, the analogue of methane, continuing with disilane Si2H6, the analogue of ethane, etc. Binary silicon-hydrogen compounds are saturated chemical compounds with the empirical formula SiHn. All contain tetrahedral silicon and terminal hydrides. They only have Si–H and Si–Si single bonds. The bond lengths are 146.0 pm for a Si–H bond and 233 pm for a Si–Si bond. The structures of the silanes are analogues of the alkanes, starting with silane, SiH4, the analogue of methane, continuing with disilane Si2H6, the analogue of ethane, etc. The simplest isomer of a silane is the one in which the silicon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for 'normal', although it is not necessarily the most common). However the chain of silicon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of silicon atoms. The members of the series (in terms of number of silicon atoms) follow: Silanes are named by adding the suffix -silane to the appropriate numerical multiplier prefix. Hence, disilane, Si2H6; trisilane Si3H8; tetrasilane Si4H10; pentasilane Si5H12; etc. The prefix is generally Greek, with the exceptions of nonasilane which has a Latin prefix, and undecasilane and tridecasilane which have mixed-language prefixes. Solid phase polymeric silicon hydrides called polysilicon hydrides are also known. When hydrogen in a linear polysilene polysilicon hydride is replaced with alkyl or aryl side-groups, the term polysilane is used. 3-Silylhexasilane (H3SiSiH2SiH(SiH3)SiH2SiH2SiH3, is the simplest chiral binary noncyclic silicon hydride. Cyclosilanes, also exist. They are structurally analogous to the cycloalkanes, with the formula SinH2n, n > 2. Early work was conducted by Alfred Stock and Carl Somiesky. Although monosilane and disilane were already known, Stock and Somiesky discovered, beginning in 1916, the next four members of the SinH2n+2 series, up to n = 6. They also documented the formation of solid phase polymeric silicon hydrides . One of their synthesis methods involved the hydrolysis of metal silicides. This method produces a mixture of silanes, which required separation on a high vacuum line. The silanes (SinH2n+2) are less thermally stable than alkanes (CnH2n+2). They tend to undergo dehydrogenation, yielding hydrogen and polysilanes. For this reason, the isolation of silanes higher than heptasilane has proven difficult. The Schlesinger process is used to prepare silanes by the reaction of perchlorosilanes with lithium aluminium hydride. The single but significant application for SiH4 is in the microelectronics industry. By metal organic chemical vapor deposition, silane is converted to silicon by thermal decomposition: