# Silicon tetrabromide

Silicon tetrabromide, also known as tetrabromosilane, is the inorganic compound with the formula SiBr$_4$. This colorless liquid has a suffocating odor due to its tendency to hydrolyze with release of hydrogen bromide. The general properties of silicon tetrabromide closely resemble those of the more commonly used silicon tetrachloride.

## Comparison of SiX$_4$

The properties of the tetrasilanes, all of which are tetrahedral, are significantly affected by nature of the halide. These trends apply also to the mixed halides. Melting points, boiling points, and bond lengths increase with the atomic mass of the halide. The opposite trend is observed for the Si-X bond energies.
| | SiH$_4$ | SiF$_4$ | SiCl$_4$ | SiBr$_4$ | SiI$_4$ |
| b.p. (˚C) | −111.9 | −90.3 | 56.8 | 155.0 | 290.0 |
| m.p. (˚C) | −185 | −95.0 | −68.8 | 5.0 | 155.0 |
| Si-X bond length (Å) | | 1.55 | 2.02 | 2.20 | 2.43 |
| Si-X bond energy (kJ/mol) | 384 | 582 | 391 | 310 | 234 |

## Lewis acidity

Covalently saturated silicon complexes like SiBr$_4$, along with tetrahalides of germanium (Ge) and tin (Sn), are Lewis acids. Although silicon tetrahalides obey the octet rule, they add Lewis basic ligands to give adducts with the formula SiBr$_4$L and SiBr$_4$L$_2$ (where L is a Lewis base). The Lewis acidic properties of the tetrahalides tend to increase as follows: SiI$_4$ < SiBr$_4$ < SiCl$_4$ < SiF$_4$. This trend is attributed to the relative electronegativities of the halogens.
The strength of the Si-X bonds decrease in the order: Si-F > Si-Cl > Si-Br > Si-I.

## Synthesis

Silicon tetrabromide is synthesized by the reaction of silicon with hydrogen bromide at 600 °C.
    Si + 4 HBr → SiBr$_4$ + 2 H$_2$
Side products include dibromosilane (SiH$_2$Br$_2$) and tribromosilane (SiHBr$_3$).
    Si + 2 HBr → SiH$_2$Br$_2$
    Si + 3 HBr → SiHBr$_3$ + H$_2$
It can also be produced by treating silicon-copper mixture with bromine:
    Si + Br$_2$ → SiBr$_4$

## Reactivity

Like other halosilanes, SiBr$_4$ can be converted to hydrides, alkoxides, amides, and alkyls, i.e., products with the following functional groups: Si-H, Si-OR, Si-NR$_2$, Si-R, and Si-X bonds respectively.
Silicon tetrabromide can be readily reduced by hydrides or complex hydrides.
    4 R$_2$AlH + SiBr$_4$ → SiH$_4$ + 4 R$_2$AlBr
Reactions with alcohols and amines proceed as follows:
    SiBr$_4$ + 4 ROH → Si(OR)$_4$ + 4 HBr
    SiBr$_4$ + 8 HNR$_2$ → Si(NR$_2$)$_4$ + 4 HNR$_2$HBr
Grignard reactions with metal alkyl halides are particularly important reactions due to their production of organosilicon compounds which can be converted to silicones.
    SiBr$_4$ + n RMgX → R$_n$SiBr$_{4−n}$ + n MgXBr
Redistribution reactions occur between two different silicon tetrahalides (as well as halogenated polysilanes) when heated to 100 ˚C, resulting in various mixed halosilanes. The melting points and boiling points of these mixed halosilanes generally increase as their molecular weights increase. (Can occur with X= H, F, Cl, Br, and I)
    2 SiBr$_4$ + 2 SiCl$_4$ → SiBr$_3$Cl + 2 SiBr$_2$Cl$_2$ + SiBrCl$_3$
    Si$_2$Cl$_6$ + Si$_2$Br$_6$ → Si$_2$Cl$_n$Br$_{6−n}$
Silicon tetrabromide hydrolyzes readily when exposed to air causing it to fume:
    SiBr$_4$ + 2 H$_2$O → SiO$_2$ + 4 HBr
Silicon tetrabromide is stable in the presence of oxygen at room temperature, but bromosiloxanes form at 670–695 ˚C .
    2 SiBr$_4$ + 1⁄2 O$_2$ → Br$_3$SiOSiBr$_3$ + Br$_2$

## Uses

Due to its close similarity to silicon tetrachloride, there are few applications unique to SiBr$_4$. The pyrolysis of SiBr$_4$ does have the advantage of depositing silicon at faster rates than that of SiCl$_4$, however SiCl$_4$ is usually preferred due to its availability in high purity. Pyrolysis of SiBr$_4$ followed by treatment with ammonia yields silicon nitride (Si$_3$N$_4$) coatings, a hard compound used for ceramics, sealants, and the production of many cutting tools.