Gallium halides

There are three sets ofgallium halides,the trihalides wheregalliumhasoxidation state+3, the intermediate halides containing gallium inoxidation states+1, +2 and +3 and some unstable monohalides, where gallium hasoxidation 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: GaF₃is 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 GaF₆octahedrasharing common corners.
GaCl₃,GaBr₃andGaI₃: These all have lower melting points than GaF₃,(GaCl₃mp 78 °C, GaBr₃mp 122 °C, GaI₃mp 212 °C) reflecting the fact that their structures all contain dimers with 4 coordinate gallium atoms and 2 bridging halogen atoms. Thus, this halides have molecular formula Ga₂Cl₆,Ga₂Br₆,Ga₂I₆,respectively. They are allLewis acids,forming mainly 4 co-ordinate adducts.GaCl₃is 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 thedichromate,Cr₂O₇²⁻,ion with two tetrahedrally coordinated gallium atoms sharing a corner. The compound can be formulated as gallium(I) heptachlorodigallate(III), Ga^IGa^III₂Cl₇.^[1]


unit cellof Ga₃Cl₇	part of thecrystal structure	structure of [Ga₂Cl₇]⁻

GaCl₂,GaBr₂and 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 waterdisproportionatingto gallium metal and gallium(III) entities. They are soluble in aromatic solvents, where arene complexes have been isolated and the arene isη⁶coordinated 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.^[2]
Ga₂Br₃and Ga₂I₃: These are formulated Ga^I₂Ga^II₂Br₆and Ga^I₂Ga^II₂I₆respectively. 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 In₂Br₃whereas the Ga₂I₆²⁻anion is isostructural with Si₂Cl₆with 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, Ga₂Br₃and 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 thatdisproportionatesabove 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 theHVPEproduction ofGaN,GaCl is produced by passing HCl gas over molten gallium which is then reacted with NH₃gas.^[3]
GaI: GaIis produced as a reactive green powder, which has been hailed as a "versatile reagent for the synthetic chemist".^[4]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]⁻.^[5]

Anionic halide complexes

Salts containing GaCl₄⁻,GaBr₄⁻and GaI₄⁻are all known. Gallium is very different fromindiumin 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

Greenwood, Norman N.;Earnshaw, Alan (1997).Chemistry of the Elements(2nd ed.).Butterworth-Heinemann.ISBN 978-0-08-037941-8.
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

^Die Kristallstruktur von Ga₃Cl₇Frank W., Hönle W., Simon A., Z. Naturforsch. Teil B (1990) 45B 1
^G.Garton; H.M.Powell (1956). "The crystal structure of gallium dichloride".Journal of Inorganic and Nuclear Chemistry.4(2): 84–89.doi:10.1016/0022-1902(57)80088-8.
^T.F. Kuech; Shulin Gu; Ramchandra Wate; Ling Zhang; Jingxi Sun; J.A. Dumesic;J.M. Redwing(2000). "The Chemistry of GaN Growth".MRS Online Proceedings Library.639.Cambridge University Press.doi:10.1557/PROC-639-G1.1.
^Baker R.J., Jones C. Dalton Trans. 2005 Apr 21;(8):1341-8
^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.

[1] Die Kristallstruktur von Ga₃Cl₇Frank W., Hönle W., Simon A., Z. Naturforsch. Teil B (1990) 45B 1

[2] G.Garton; H.M.Powell (1956). "The crystal structure of gallium dichloride".Journal of Inorganic and Nuclear Chemistry.4(2): 84–89.doi:10.1016/0022-1902(57)80088-8.

[3] T.F. Kuech; Shulin Gu; Ramchandra Wate; Ling Zhang; Jingxi Sun; J.A. Dumesic;J.M. Redwing(2000). "The Chemistry of GaN Growth".MRS Online Proceedings Library.639.Cambridge University Press.doi:10.1557/PROC-639-G1.1.

[4] Baker R.J., Jones C. Dalton Trans. 2005 Apr 21;(8):1341-8

[WiddifieldJurca2012-5] 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.

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