Gallium(III) bromide

Gallium(III) bromide is, at room temperature and atmospheric pressure, a white, crystalline powder which reacts favorably and exothermically with water.[1] Solid gallium tribromide is stable at room temperature and can be found primarily in its dimeric form.[2] GaBr₃ can form an intermediate halide, Ga₂Br_7; however, this is not as common as with GaCl₃. It is a member of the gallium trihalide group and is similar to GaCl₃, and GaI₃, but not GaF₃, in its preparation and uses.[2] GaBr₃ is a milder Lewis acid than AlBr₃, and has more versatile chemistry due to the comparative ease of reducing gallium, but is more reactive than GaCl₃.[3]

GaBr₃ is similar spectroscopically to aluminum, indium, and thallium trihalides excluding trifluorides.[4]

One method of preparing GaBr₃ is to heat elemental gallium in the presence of bromine liquid under vacuum.[5] Following the highly exothermic reaction, the mixture is allowed to rest and then subjected to various purifying steps. This method from the turn of the twentieth century remains a useful way of preparing GaBr_3. Historically Gallium was obtained by electrolysis of its hydroxide in KOH solution, however today it is obtained as a byproduct of Aluminum and Zinc production.

GaBr₃ can be synthesized by exposing gallium metal to elemental bromine in a water, oxygen, organic, and grease-free environment.[5][6] The result is a gas which must be crystallized in order to form the GaBr₃ solid purchased by laboratories. Below is the equation:

2 Ga(s) + 3 Br₂(l) ⟶ 2 GaBr₃(g)

Extended crystal structure of GaBr₃

The GaBr₃ monomer has trigonal planar geometry, but when it forms the dimer Ga₂Br₆ the geometry around the Gallium center distorts to become roughly tetrahedral. As a solid, GaBr₃ forms a monoclinic crystalline structure with a unit cell volume of 524.16 ų. Additional specifications for this unit cell are as follows: a = 8.87 Å, b = 5.64 Å, c =11.01 Å, α = 90˚, β = 107.81˚, γ = 90˚.[7]

Gallium is the lightest Group 13 metal with a filled d-shell, and has an electronic configuration of ([Ar] 3d¹⁰ 4s² 4p¹) below the valence electrons that could take part in d-π bonding with ligands. The somewhat high oxidation state of Ga in Ga(III)Br₃, low electronegativity, and high polarizability allow GaBr₃ to behave as a "soft acid" in terms of the Hard-Soft-Acid-Base (HSAB) theory. The Lewis acidity of all the gallium trihalides, GaBr₃ included, has been extensively studied thermodynamically, and the basicity of GaBr₃ has been established with a number of donors.[2]

GaBr₃ is capable of accepting an additional Br⁻ ion or unevenly splitting its dimer to form [GaBr₄]⁻, a tetrahedral ion of which crystalline salts can be obtained.[2][8] This ionic complex is further capable of binding to . The Br⁻ ion can be just as easily substituted with a neutral ligand. Typically these neutral ligands, with form GaBr₃ L and sometimes GaBr₃L₂, will form a tetrahedral bipyramidal geometric structure with the Br in an equatorial position due to their large effective nuclear charge.[2] Additionally, GaBr₃ can be used as a catalyst in certain oxidative addition reactions.

GaBr₃ is used as a catalyst in organic synthesis, with similar mechanism to GaCl_3. However, due to its greater reactivity, it is sometimes disfavored because of the greater versatility of GaCl₃.[2] GaBr₃ as well as other gallium trihalides and group 13 metal trihalides can be used as catalysts in the oxidative addition of organic compounds. It has been verified that the GaBr₃ dimer cleaves unevenly into [GaBr₄]⁻ and [GaBr₂]⁺.[8] The entire mechanism is uncertain partly because intermediate states are not always stable enough for study, and partially because GaBr₃ is studied less frequently than GaCl₃. Ga(III) itself is a useful Lewis acid for organic reactions because its full d-electron shell makes it able to accept variable numbers of ligands, but will readily give up ligands if conditions prove favorable.

• GaCl₃
• Catalysis