Silanes

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Chemical structure of silane, the simplest silane. Notice that the silicon atom is located at the center of the silane tetrahedron.

Silanes are saturated chemical compounds with the formula RmSiX4-m or oligomeric derivatives (R = organic substituents, H; X = alkoxy, halide). This article is focused on hydrides alone with the formula SixHy where Si has tetrahedral molecular geometry. The simplest silane (the parent molecule) is silane, SiH
4
. One homologous series of silanes are analogues of the alkanes.

Saturated hydrosilicons can be:

  • linear or branched (general formula SinH2n + 2) wherein the silicon atoms are joined in a snake-like structure
  • monocyclic (general formula SinH2n, n > 2) wherein the silicon backbone is linked so as to form a loop.

According to the definition by IUPAC, the former two are silanes, whereas the third group is called cyclosilanes. Saturated hydrosilicons can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still silanes (no general formula) as long as they are acyclic (i.e., having no loops). They also have single covalent bonds between their silicons.

Linear silanes[edit]

Straight-chain silanes are sometimes indicated by the prefix n- (for normal) for a non-linear isomer exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers.

Acyclic silanes with more than three silicon atoms can be arranged in various ways, forming structural isomers. 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:

silane, SiH
4
- one silicon and four hydrogen
disilane, Si
2
H
6
- two silicon and six hydrogen
trisilane, Si
3
H
8
- three silicon and 8 hydrogen
tetrasilane, Si
4
H
10
- four silicon and 10 hydrogen (two isomers: tetrasilane & isotetrasilane)
pentasilane, Si
5
H
12
- five silicon and 12 hydrogen (three isomers: pentasilane, isopentasilane & neopentasilane)

Silanes are named by adding the suffix -silane to the appropriate numerical multiplier prefix. Hence, disilane, Si
2
H
6
; trisilane Si
3
H
8
; tetrasilane Si
4
H
10
; pentasilane Si
5
H
12
; 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.

3-Silylhexasilane is the simplest chiral binary silicon hydride.

Circular structures, called cyclosilanes, also exist. These are analogous to the cycloalkanes. 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.

Unsaturated silicon hydride molecules called silenes contain the doubly bonded silicon atom, >Si= (e.g. methylene silane or silaethene, H2Si(CH2)). They cannot be isolated under normal conditions. The silylenes contain the divalent Si atom (e.g. SiH2, SiF2). Most silylenes are unstable at normal temperatures and can only be synthesized in the gas phase at elevated temperatures or isolated in a low-temperature matrix.

Physical properties[edit]

Table of silanes
Silane Formula Boiling point [°C] Melting point [°C] Density [g cm−3] (at 25 °C)
Silane SiH
4
−112 −185 gas
Disilane Si
2
H
6
−14 −132 gas
Trisilane Si
3
H
8
53 −117 0.743 (liquid)
Tetrasilane Si
4
H
10
100 −84 liquid


History[edit]

Early work was conducted by Alfred Stock and Carl Somiesky.[1] 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, and they also documented the formation of solid phase polymeric silicon hydrides (see below).[2] Usually polymeric silanyl hydrides are based on silicone and thus called silicone hydrides (the back bone consists of Si–O–Si bonds).

Chemical properties[edit]

The silanes (SinH2n+2) are much less thermally stable than alkanes (CnH2n+2) and they are kinetically labile, with their decomposition reaction rate increasing with increases in the number of silicon atoms in the molecule. This makes preparation and isolation of SinH2n+2 molecules with n greater than about 8 difficult.[3] Greater catenation of the Si atoms can be obtained with the halides (SinX2n+2 with n = 14 for the fluorides) because of pi back bonding from the halogen p orbitals to the Si d orbitals, which compensates for the electron withdrawal from Si towards the halogen that occurs through the sigma bonding.[4]

Silanes can also incorporate the same functional groups as alkanes, e.g. –OH, to make a silanol (an analogue of alcohol) or a halogen to make a silicon halide (an analogue of alkyl halide). There is (in principle) a silicon analogue for all carbon alkanes derivatives. Like carbon, there also exists silicocations, an important class is the silanyliums, of which simplest member is the silylium ion (SiH+
3
).

Production[edit]

Schlesinger[edit]

The Schlesinger process is a method to synthesise liquid silanes, from perchlorosilanes and lithium tetrahydridoaluminate(-1).

Laboratory preparation[edit]

There is usually little need for silanes to be synthesised in the laboratory, since they are usually commercially available. Silanes are generally reactive chemically and biologically, but do not undergo functional group interconversions cleanly. When silanes are produced in the laboratory, it is often a side-product of a reaction. For example, the use magnesium silicide as a base gives the conjugate acids, mixed silanes as a side-product.

In general, the alkaline-earth metals form silicides with various stoichiometry. However, in all cases these substances react with Brønsted–Lowry acids to produce some type of hydride of silicon that is dependent on the Si anion connectivity in the silicide. The possible products include silane and/or higher molecules in the homologous series, a polymeric silicon hydride, or a silicic acid. Hence, MSi with their zigzag chains of Si anions (containing two lone pairs of electrons on each Si anion that can accept protons) yield the polymeric hydride (SiH2)x.

However, at times it may be desirable to make a portion of a molecule into a silane like functionality (silanyl group) using the above or similar methods.

Applications[edit]

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[edit]

Silane is explosive when mixed with air (1 – 98% SiH4). 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 don't react spontaneously 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:

  • Silane is slightly heavier than air (possibility of pooling at ground levels/pits)
  • Disilane is heavier than air (possibility of pooling at ground levels/pits)
  • Trisilane is heavier than air (possibility of pooling at ground levels/pits)

Nomenclature[edit]

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[edit]

  1. ^ E. Wiber, Alfred Stock and the Renaissance of Inorganic Chemistry," Pure Appl. Chem., Vol. 49 (1977) pp. 691-700.
  2. ^ J. W. Mellor, "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VI, Longman, Green and Co. (1947) pp. 223 - 227.
  3. ^ W. W. Porterfield "Inorganic Chemistry: A Unified Approach," Academic Press (1993) p. 219.
  4. ^ A. Earnshaw, N. Greenwood, "Chemistry of the Elements," Butterworth-Heinemnann (1997) p. 341.