Gallium trichloride
Gallium trichloride is the chemical compound with the formula GaCl. Solid gallium trichloride exists as a dimer with the formula GaCl. 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, GaCl 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. GaCl is known whereas the corresponding AlCl 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. It can also be prepared from by heating gallium oxide with thionyl chloride: 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 AlCl and InCl 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 GaCl is often written as Ga(μ-Cl)Cl. 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] 3"d"4"s"4"p") below the valence electrons that could take part in "d"-π bonding with ligands. The low oxidation state of Ga in Ga(III)Cl, along with the low electronegativity and high polarisability, allow GaCl 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: With a chloride ion as ligand the tetrahedral GaCl ion is produced, the 6 coordinate GaCl cannot be made. Compounds like KGaCl that have a chloride bridged anion are known. In a molten mixture of KCl and GaCl, the following equilibrium exists: 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.