Gallium(III) bromide

Gallium(III) bromide (GaBr₃) is a chemical compound and one of four gallium trihalides.

Introduction

At room temperature and atmospheric pressure, Gallium(III) bromide is a white, crystalline powder that reacts favorably and exothermically with water. Solid gallium tribromide is stable at room temperature and can be found primarily in its dimeric form. 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. GaBr₃ is a milder Lewis acid than AlBr₃ and has more versatile chemistry due to the comparative ease of reducing gallium, but it is more reactive than GaCl₃.

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

Preparation

One method of preparing GaBr₃ is to heat elemental gallium in the presence of bromine liquid under vacuum. Following the highly exothermic reaction, the mixture is allowed to rest and is 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 solution of potassium hydroxide. Today, it is obtained as a byproduct of aluminum and zinc production.

GaBr₃ can be synthesized by exposing gallium to bromine in an environment free of water, oxygen and grease. The result is a gas which must be crystallized in order to form bromide purchased by laboratories. Below is the equation:

Structure

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˚.

Complexes

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.

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. 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. Additionally, GaBr₃ can be used as a catalyst in certain oxidative addition reactions.

Uses

GaBr₃ is used as a catalyst in organic synthesis, with a similar mechanism to GaCl_3. However, due to its greater reactivity, it is sometimes disfavored because of the greater versatility of GaCl₃. 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]⁺. 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 it will readily give up ligands if conditions prove favorable.

See also

• GaCl₃
• Catalysis

References