Binary silicon-hydrogen compounds

Chemical structure of disilane, which is structurally similar to ethane.

Silanes are saturated chemical compounds with the empirical formula Si_xH_y. They are hydrosilanes, a class of compounds that includes compounds with Si−H and other Si−X bonds. 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. They are mainly of theoretical or academic interest.^[1]

Inventory

Cyclopentasilane is structurally similar to cyclopentane, just larger.

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 silicon atoms. 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:

silane, SiH₄, 1 silicon atom and 4 hydrogen atoms, analogous to methane
disilane, Si₂H₆ or H₃Si−SiH₃, 2 silicon atoms and 6 hydrogen atoms, analogous to ethane
trisilane, Si₃H₈ or H₂Si(−SiH₃)₂, 3 silicon atoms and 8 hydrogen atoms, analogous to propane
tetrasilane, Si₄H₁₀ or H₃Si−SiH₂−SiH₂−SiH₃, 4 silicon atoms and 10 hydrogen atoms, analogous to butane (one isomer: isotetrasilane, analogous to isobutane)
pentasilane, Si₅H₁₂ or H₃Si−SiH₂−SiH₂−SiH₂−SiH₃, 5 silicon atoms and 12 hydrogen atoms, analogous to pentane (two isomers: isopentasilane H₃Si−SiH₂−SiH(−SiH₃)₂ and neopentasilane Si(SiH₃)₄, analogous to isopentane and neopentane, respectively)
hexasilane Si₆H₁₄ or H₃Si−SiH₂−SiH₂−SiH₂−SiH₂−SiH₃, 6 silicon atoms and 14 hydrogen atoms, analogous to hexane

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, H₃Si−SiH₂−SiH(−SiH₃)−SiH₂−SiH₂−SiH₃, is the simplest chiral binary noncyclic silicon hydride.

Cyclosilanes also exist. They are structurally analogous to the cycloalkanes, with the formula Si_nH_2n, n > 2.

Data for small silanes^[1]
Silane	Formula	Melting point [°C]	Boiling point [°C]	Density [g cm⁻³] (at 25 °C)	Appearance
Silane	SiH₄	−185	−112		Colorless gas
Disilane	Si₂H₆	−132	−14		Colorless gas
Trisilane	Si₃H₈	−117	53	0.743	Colorless liquid
Cyclotrisilane [el]	Si₃H₆
Tetrasilane [de; ru]	Si₄H₁₀	−90	108	0.793	Colorless liquid
Pentasilane [de; ru]	Si₅H₁₂	−72.8	153	0.827	Colorless liquid
Cyclopentasilane	Si₅H₁₀	−10.5^{[clarification needed]}	194^{[clarification needed]}	0.963	Colorless liquid
Hexasilane [de; ru]	Si₆H₁₄	−44.7	193.6	0.847	Colorless liquid

Production

Early work was conducted by Alfred Stock and Carl Somiesky.^[2] Although monosilane and disilane were already known, Stock and Somiesky discovered, beginning in 1916, the next four members of the Si_nH_2n+2 series, up to n = 6. They also documented the formation of solid phase polymeric silicon hydrides.^[3] 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.^[4]^[5]^[6]

The silanes (Si_nH_2n+2) are less thermally stable than alkanes (C_nH_2n+2). They tend to undergo dehydrogenation, yielding hydrogen and polysilanes. For this reason, the isolation of silanes higher than heptasilane has proved difficult.^[7]

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:

SiH₄ → Si + 2 H₂

Hazards

Silane is explosive when mixed with air (1 – 98% SiH₄^{[clarification needed]}). Other lower silanes can also form explosive mixtures with air. The lighter liquid silanes are highly flammable; this risk increases with the length of the silicon chain.

Considerations for detection/risk control:

Silane is slightly denser than air, while disilane and trisilane are denser than air, thus there is a possibility of pooling at ground levels/pits.

Nomenclature

The IUPAC nomenclature (systematic way of naming compounds) 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:

Identify the longest continuous chain of silicon atoms
Name this longest root chain using standard naming rules
Name each side chain by changing the suffix of the name of the silane from "-ane" to "-anyl", except for "silane" which becomes "silyl"
Number the root chain so that the sum of the numbers assigned to each side group will be as low as possible
Number and name the side chains before the name of the root chain

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.

References

^ ^a ^b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ E. Wiberg, Alfred Stock and the Renaissance of Inorganic Chemistry," Pure Appl. Chem., Vol. 49 (1977) pp. 691-700.
^ J. W. Mellor, "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VI, Longman, Green and Co. (1947) pp. 223 - 227.
^ Hydrides of Boron and Silicon. Ithaca (USA) 1933.
^ Stock, A.; Stiebeler, P.; Zeidler, F. (1923). "Siliciumwasserstoffe, XVI. Die höheren Siliciumhydride". Ber. Dtsch. Chem. Ges. B. 56B: 1695–1705. doi:10.1002/cber.19230560735.
^ P. W. Schenk (1963). "Silanes SiH₄ (Si₂H₆, Si₃H₈)". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. Vol. 1. NY, NY: Academic Press. pp. 679–680.
^ W. W. Porterfield "Inorganic Chemistry: A Unified Approach," Academic Press (1993) p. 219.

[G&E-1] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[2] E. Wiberg, Alfred Stock and the Renaissance of Inorganic Chemistry," Pure Appl. Chem., Vol. 49 (1977) pp. 691-700.

[3] J. W. Mellor, "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VI, Longman, Green and Co. (1947) pp. 223 - 227.

[4] Hydrides of Boron and Silicon. Ithaca (USA) 1933.

[5] Stock, A.; Stiebeler, P.; Zeidler, F. (1923). "Siliciumwasserstoffe, XVI. Die höheren Siliciumhydride". Ber. Dtsch. Chem. Ges. B. 56B: 1695–1705. doi:10.1002/cber.19230560735.

[6] P. W. Schenk (1963). "Silanes SiH₄ (Si₂H₆, Si₃H₈)". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. Vol. 1. NY, NY: Academic Press. pp. 679–680.

[7] W. W. Porterfield "Inorganic Chemistry: A Unified Approach," Academic Press (1993) p. 219.

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