Gallium trichloride

Gallium trichloride is the chemical compound with the formula GaCl₃. Solid gallium trichloride exists as a dimer with the formula Ga₂Cl₆. It is colourless and soluble in virtually all solvents, even alkanes, which is truly unusual for a metal halide. It is the main precursor to most derivatives of gallium and a reagent in organic synthesis.

As a Lewis acid, GaCl₃ is milder than aluminium trichloride. Gallium(III) is easier to reduce than Al(III), so the chemistry of reduced gallium compounds is more extensive than for aluminium. Ga₂Cl₄ is known whereas the corresponding Al₂Cl₄ is not. The coordination chemistry of Ga(III) and Fe(III) are similar, and gallium(III) compounds have been used as diamagnetic analogues of ferric compounds.

Preparation

Gallium trichloride can be prepared from the elements, heating gallium metal in a stream of chlorine, and purifying the product by sublimation under vacuum.

:2 Ga + 3 Cl₂ → 2 GaCl₃

It can also be prepared from by heating gallium oxide with thionyl chloride:
:Ga₂O₃ + 3 SOCl₂ → 2 GaCl₃ + 3 SO₂

Gallium metal reacts slowly with hydrochloric acid. This reaction produces hydrogen gas slowly.

Structure

As a solid, it adopts a bitetrahedral structure with two bridging chlorides. Its structure resembles that of aluminium tribromide. In contrast AlCl₃ and InCl₃ feature contain 6 coordinate metal centers. As a consequence of its molecular nature and associated low lattice energy, gallium trichloride has a lower melting point vs the aluminium and indium trihalides. The formula of Ga₂Cl₆ is often written as Ga₂(μ-Cl)₂Cl₄. In the gas phase the dimers dissociate to trigonal planar monomers.

Complexes

Gallium is the lightest member of Group 13 to have a full ''d'' shell, (gallium has the electronic configuration Ar.html" ;"title="Argon.html" ;"title="nowiki/>Argon">Ar">Argon.html" ;"title="nowiki/>Argon">Ar3''d''¹⁰ 4''s''² 4''p''¹) below the valence electrons that could take part in ''d''-π bonding with ligands.
The low oxidation state of Ga in Ga(III)Cl₃, along with the low electronegativity and high polarisability, allow GaCl₃ to behave as a "soft acid" in terms of the HSAB theory. The strength of the bonds between gallium halides and ligands have been extensively studied. What emerges is:
*GaCl₃ is a weaker Lewis acid than AlCl₃ towards N and O donors e.g. pyridine
*GaCl₃ is a stronger Lewis acid than AlCl₃ towards thioethers e.g. dimethyl sulfide, Me₂S
With a chloride ion as ligand the tetrahedral GaCl₄⁻ ion is produced, the 6 coordinate GaCl₆³⁻ cannot be made. Compounds like KGa₂Cl₇ that have a chloride bridged anion are known.J H von Barner Inorg Chem 1985 24 1686
In a molten mixture of KCl and GaCl₃, the following equilibrium exists:
:2 GaCl₄⁻ Ga₂Cl₇⁻ + Cl⁻

Uses

Organic synthesis

Gallium trichloride is a Lewis acid catalyst, such as in the Friedel–Crafts reaction, and is also used in carbogallation reactions of compounds with a carbon-carbon triple bond. It is a precursor to organogallium reagents. It is also used as a catalyst in many organic reactions.

Detection of solar neutrinos

110 tons of gallium trichloride aqueous solution was used in the GALLEX and GNO experiments performed at Laboratori Nazionali del Gran Sasso in Italy to detect solar neutrinos. In these experiments, germanium-71 was produced by neutrino interactions with the isotope gallium-71 (which has a natural abundance of 40%), and the subsequent beta decays of germanium-71 were measured.David R. Lide, ed. ''Handbook of Chemistry and Physics, 85th Edition'', Internet Version 2005. CRC Press, 2005.

See also

* Gallium halides

References

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External links

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{{DEFAULTSORT:Gallium Trichloride
Inorganic compounds
Gallium compounds
Chlorides
Metal halides