View source for Gallium halides

There are three sets of '''gallium halides''', the trihalides where gallium has [[oxidation state|oxidation state]] +3, the intermediate halides containing gallium in [[oxidation state|oxidation states]] +1, +2 and +3 and some unstable monohalides, where gallium has [[oxidation state|oxidation state]] +1. 
==Trihalides==
All four trihalides are known. They all contain gallium in the +3 oxidation state. Their proper names are gallium(III) fluoride, gallium(III) chloride, gallium(III) bromide and gallium(III) iodide.
;GaF<sub>3</sub>
:GaF<sub>3</sub> is a white solid which sublimes before it melts, mp estimated as above 1000<sup>o</sup>C. It contains 6 co-ordinate gallium atoms with a three-dimensional network of GaF<sub>6</sub> octahedra sharing common corners.
;[[Gallium_trichloride|GaCl<sub>3</sub>]], GaBr<sub>3</sub> and GaI<sub>3</sub> 
:These all have lower melting points than GaF<sub>3</sub>, ([[Gallium_trichloride|GaCl<sub>3</sub>]] mp 78<sup>o</sup>C, GaBr<sub>3</sub> mp 122<sup>o</sup>C, GaI<sub>3</sub> mp 212<sup>o</sup>C) reflecting the fact that their structures  all contain dimers with 4 coordinate gallium atoms and 2 bridging halogen atoms. They are all [[Lewis_acid|Lewis acids]], forming mainly 4 co-ordinate adducts. [[Gallium_trichloride|GaCl<sub>3</sub>]] is the most commonly used trihalide.

==Intermediate halides==
Intermediate chlorides, bromides and iodides exist. They contain gallium in oxidation states +1, +2 and +3.
;GaCl<sub>2</sub>, GaBr<sub>2</sub> and GaI<sub>2</sub> 
:These are the best known and most studied intermediate halides. They contain gallium in oxidation states +1 and +3 and are formulated Ga<sup>I</sup>Ga<sup>III</sup>X<sub>4</sub>. The dihalides are unstable in the presence of water disproportionating to gallium metal and gallium(III) entities. They are soluble in aromatic solvents, where arene complexes have been isolated and the arene is [[hapticity|η<sup>6</sup>]] coordinated to the Ga<sup>+</sup> ion. With some ligands, L, e.g. dioxane, a neutral complex, Ga<sub>2</sub>X<sub>2</sub>L<sub>2</sub>, with a gallium-gallium bond is produced.  These compounds have been used as a route into gallium chain and cluster compounds. <br/>
;Ga<sub>2</sub>Br<sub>3</sub> and Ga<sub>2</sub>I<sub>3</sub>
:These are formulated Ga<sup>I</sup><sub>2</sub> Ga<sup>II</sup><sub>2</sub>Br<sub>6</sub> and Ga<sup>I</sup><sub>2</sub> Ga<sup>II</sup><sub>2</sub>I<sub>6</sub> respectively. Both anions contain a gallium-gallium bond where gallium has a formal oxidation state of +2. The Ga<sub>2</sub>Br<sub>6</sub><sup>2-</sup> anion is eclipsed like the In<sub>2</sub>Br<sub>6</sub><sup>2-</sup> anion in In<sub>2</sub>Br<sub>3</sub> whereas the Ga<sub>2</sub>I<sub>6</sub><sup>2-</sup> anion is isostructural with Si<sub>2</sub>Cl<sub>6</sub> with a staggered conformation.

==Monohalides==
None of the monohalides are stable at room temperature. The previously reported GaBr and GaI produced from fusing gallium with the trihalide have been shown to be gallium metal rich Ga<sub>2</sub>Br<sub>3</sub> and Ga<sub>2</sub>I<sub>3</sub> respectively.<br/>
;GaCl and GaBr
:GaCl and GaBr have been produced in the gas form from the reaction of HX and molten gallium using a special reactor. They have been isolated by quenching the high temperature gas at 77<sup>o</sup>K. GaCl is reported as a red solid that disproportionates above 0<sup>o</sup>C. Both GaCl and GaBr produced in this way can be stabilised in suitable solvents. The metastable solutions formed in this way have been used as precursors to numerous gallium cluster compounds.<br>
:In the [[Hydride_vapour_phase_epitaxy|HVPE]] production of [[Gallium(III)_nitride|GaN]], GaCl is produced by passing HCl gas over molten gallium which is then reacted with NH<sub>3</sub> gas.  <ref> Kuech T.F, Shulin Gu, Ramchandra Wate, Ling Zhang, Jingxi Sun, J.A. Dumesic, and J.M. Redwing Mat. Res. Soc. Symp. Proc. Vol. 639 G 1.1.1</ref><br/>
;GaI
:The chemical structure of the reagent termed ‘GaI’ produced from reacting gallium metal with iodine in toluene using ultrasound is not known. GaI is produced as a reactive green powder, which has been hailed as a “versatile reagent for the synthetic chemist”. <ref> Baker RJ, Jones C. Dalton Trans. 2005 Apr 21;(8):1341-8</ref> <br/>

==Anionic halide complexes==
Salts containing GaCl<sub>4</sub><sup>-</sup>, GaBr<sub>4</sub><sup>-</sup> and GaI<sub>4</sub><sup>-</sup> are all known. Gallium is very different from [[Indium_halides|indium]] in that it is only known to form  6 coordinate complexes with the fluoride ion. This can be rationalised by the smaller size of gallium (ionic radii of Ga(III) 62 pm, In(III) 80 pm).<br/>
Salts containing the Ga<sub>2</sub>Cl<sub>6</sub><sup>2-</sup> anion, where gallium has an oxidation state of +2, are known.

== General references ==
* [http://www.webelements.com/ WebElements]
#{{Greenwood&Earnshaw}}
#{{Cotton&Wilkinson6th}}

== Footnotes ==
{{reflist|2}}
[[Category:Gallium compounds]]
[[Category:Metal halides]]
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