Binary silicon-hydrogen compounds


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,, the analogue of methane, continuing with disilane, the analogue of ethane, etc.

Inventory

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. 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 follow:
Silanes are named by adding the suffix -silane to the appropriate numerical multiplier prefix. Hence, disilane, ; trisilane ; tetrasilane ; pentasilane ; 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 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.

Production

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 are less thermally stable than alkanes. 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.

Applications

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:

Hazards

Silane is explosive when mixed with air. Other lower silanes can also form explosive mixtures with air. The lighter liquid silanes are highly flammable, but this risk decreases with the length of the silicon chain as was discovered by Peter Plichta. Silanes above Heptasilane and can be stored like gasoline. Higher silanes have therefore the potential to replace hydrocarbons as storable energy source with the advantage to react not only with oxygen but also with nitrogen.
Considerations for detection/risk control:
The IUPAC nomenclature for silanes is based on identifying hydrosilicon chains. Unbranched, saturated hydrosilicon chains are named systematically with a Greek numerical prefix denoting the number of silicons and the suffix "-silane".
IUPAC naming conventions can be used to produce a systematic name.
The key steps in the naming of more complicated branched silanes are as follows:
The nomenclature parallels that of alkyl radicals.
Silanes can also be named like any other inorganic compound; in this naming system, silane is named silicon tetrahydride. However, with longer silanes, this becomes cumbersome.