Gallium trichloride

Gallium trichloride is the chemical compound with the formula GaCl₃. Solid gallium trichloride exists as a dimer with the formula Ga₂Cl₆.^[1] It is colourless and soluble in virtually all solvents, which is unusual for a metal halide. It is the main precursor to most derivatives of gallium and a reagent in organic synthesis.^[2]

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.

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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.^[3][4]

Structure

The dimeric structure involving two chlorine bridging atoms with the gallium atoms essentially tetrahedrally coordinated by chlorine is surprising, as the chlorides of the two members of group 13 above and below gallium, AlCl₃ and InCl₃ both contain 6 coordinate metal atoms in a three dimensional structure. As a consequence of its structure where there are no significant lattice forces, gallium trichloride has the lowest melting point of all of the aluminium, gallium 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 3d10 4s2 4p1) below the valence electrons that could take part in d-π bonding with ligands. This in terms, hard and soft acid-base theory allows gallium(III) to behave as "soft" acid. 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^[5] In a molten mixture of KCl and GaCl₃, the following equilibrium exists:

2 GaCl₄⁻ Ga₂Cl₇⁻ + Cl⁻

Use in detection of solar neutrinos

110 tons of gallium trichloride solution has been used in GALLEX experiment performed by Laboratori Nazionali del Gran Sasso in Italy to detect solar neutrinos. In this experiment, isotope germanium-71 is produced and being measured.^[6]

See also

• Gallium halides

References

• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
• Gallium. WebElements Periodic Table.

1. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
2. ^ Yamaguchi, M.; Shibasaki, M. Gallium Trichloride" in Encyclopedia of Reagents for Organic Synthesis 2005; John Wiley & Sons. doi:10.1002/047084289X.rn00118u
3. ^ S.C. Wallwork I.J.Worral J.Chem. Soc 1965,1816
4. ^ Kovar, R. A. "Gallium Trichloride" Inorganic Syntheses, 1977, volume XVII, pp 167-172. ISBN 0-07-044327-0
5. ^ J H von Barner Inorg Chem 1985 24 1686
6. ^ David R. Lide, ed. Handbook of Chemistry and Physics, 85th Edition, Internet Version 2005. CRC Press, 2005.