Gallium halides

There are three sets of gallium halides, the trihalides where gallium has oxidation state +3, the intermediate halides containing gallium in oxidation states +1, +2 and +3 and some unstable monohalides, where gallium has 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.

GaF₃

GaF₃ is a white solid which sublimes before it melts, with an estimated melting point above 1000 °C. It contains 6 co-ordinate gallium atoms with a three-dimensional network of GaF₆ octahedra sharing common corners.

GaCl₃, GaBr₃ and GaI₃

These all have lower melting points than GaF₃, (GaCl₃ mp 78 °C, GaBr₃ mp 122 °C, GaI₃ mp 212 °C) reflecting the fact that their structures all contain dimers with 4 coordinate gallium atoms and 2 bridging halogen atoms. They are all Lewis acids, forming mainly 4 co-ordinate adducts. GaCl₃ is the most commonly used trihalide.

Intermediate halides

Intermediate chlorides, bromides and iodides exist. They contain gallium in oxidation states +1, +2 and +3.

Ga₃Cl₇

This compound contains the Ga₂Cl₇⁻ ion, which has a structure similar to the dichromate, Cr₂O₇²⁻, ion with two tetrahedrally coordinated gallium atoms sharing a corner. The compound can be formulated as gallium(I) heptachlorodigallate(III), Ga^I Ga^III₂Cl₇.^[1]

unit cell of Ga3Cl7	part of the crystal structure	structure of [Ga2Cl7]^−

GaCl₂, GaBr₂ and GaI₂

These are the best known and most studied intermediate halides. They contain gallium in oxidation states +1 and +3 and are formulated Ga^IGa^IIIX₄. 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 η⁶ coordinated to the Ga⁺ ion. With some ligands, L, e.g. dioxane, a neutral complex, Ga₂X₂L₂, with a gallium-gallium bond is produced. These compounds have been used as a route into gallium chain and cluster compounds.

Ga₂Br₃ and Ga₂I₃

These are formulated Ga^I₂ Ga^II₂Br₆ and Ga^I₂ Ga^II₂I₆ respectively. Both anions contain a gallium-gallium bond where gallium has a formal oxidation state of +2. The Ga₂Br₆²⁻ anion is eclipsed like the In₂Br₆²⁻ anion in In₂Br₃ whereas the Ga₂I₆²⁻ anion is isostructural with Si₂Cl₆ 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 mixtures of metallic gallium with, respectively, Ga₂Br₃ and Ga₂I₃.

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 K. GaCl is reported as a red solid that disproportionates above 0 °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.

In the HVPE production of GaN, GaCl is produced by passing HCl gas over molten gallium which is then reacted with NH₃ gas.^[2]

GaI

GaI is produced as a reactive green powder, which has been hailed as a "versatile reagent for the synthetic chemist".^[3] The chemical structure of the reagent termed ‘GaI’ produced from reacting gallium metal with iodine in toluene using ultrasound has only recently been investigated using 69/71Ga solid-state NMR and a tentative structure assigned which includes gallium metal atoms, [Ga⁰]₂[Ga]⁺[GaI₄]⁻.^[4]

Anionic halide complexes

Salts containing GaCl₄⁻, GaBr₄⁻ and GaI₄⁻ are all known. Gallium is very different from 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).
Salts containing the Ga₂Cl₆²⁻ anion, where gallium has an oxidation state of +2, are known.

General references

• WebElements

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
2. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5

Footnotes

1. Die Kristallstruktur von Ga₃Cl₇ Frank W., Hönle W., Simon A., Z. Naturforsch. Teil B (1990) 45B 1
2. 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
3. Baker R.J., Jones C. Dalton Trans. 2005 Apr 21;(8):1341-8
4. Widdifield, Cory M.; Jurca, Titel; Richeson, Darrin S.; Bryce, David L. (2012). "Using 69/71Ga solid-state NMR and 127I NQR as probes to elucidate the composition of "GaI"". Polyhedron. 35 (1): 96–100. doi:10.1016/j.poly.2012.01.003. ISSN 0277-5387.