Aluminium trihalides

AIF3 is made by treating AI2O3 with HF gas at 700° and the other trihalides are made by the direct exothermic combination of the elements. AIF3 is important in the industrial production of A1 metal (p. 219) and is made on a scale approaching 700000 tonnes per annum world wide. AICI3 finds extensive use as a Friedel-Crafts catalyst (p. 236) its annual production approaches lOOOOOtpa and is dominated by Western Europe, USA and Japan. The price for bulk AICI3 is about 0.35/kg. 

The structure of AICI3 is similarly revealing. The crystalline solid has a layer lattice with 6-coordinate Al but at the mp 192.4° the stmcture changes to a 4-coordinate molecular dimer Al2Clg as a result there is a dramatic increase in volume (by 85%) and an even more dramatic drop in electrical conductivity almost to zero. The mp therefore represents a substantial change in the nature of the bonding. The covalently bonded 

Structural Inorganic Chemistry, 4th edn., Oxford University Press, Oxford, 1975, 1095 pp. 

Hargittai, M. Kolonits, J. Tremmel, J.-L. Four-QUET and G. Ferey, Struct. Chem. 1, 75-8 (1989). 

The Irihalides of A1 form a large number of addition compounds or complexes and these have been extensively studied because of their importance in understanding the nature of Friedel-Crafts catalysis. The adducts vary enormously in stability from weak interactions to very stable complexes, and they also vary widely in their mode of bonding, structure and 

The stmcture of AICI3 is similarly revealing. The crystalline solid has a layer lattice with 

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Three main structural sub-groups can be recognised alkylaluminium dihalides, dialkylaluminium halides, and trialkyldialuminium trihalides (equimolar complexes of a trialkylaluminium and an aluminium trihalide). While this is generally a very reactive group of compounds, similar in reactivity to trialkylaluminium compounds, increase in size of the alkyl groups present and in the degree of halogen substitution tends to reduce pyrophoricity. 

In the case of 87e and 87f insertion of a second equivalent of phosphinidene complex [(CO)5W-PCH(TMS)2] into a P-C or P-P bond of the three-membered ring has led to l,2-dihydro-l,2,3 triphosphete complexes lOOe and lOOf. The reactions of aluminium trihalides and gallium trichloride with 3 equiv of kinetically stabilized phosphaalkynes P CR proceed according to an unusual spirocyclotrimerization to provide the complexes 103a-103f exhibiting structural features of a zwitterionic diphosphirenium alkanide complex (Scheme 37). 

Molecules such as boron trihalide or aluminium trihalide have an empty orbital on the central atom, because in each case there are only three electron pairs around it. Electrophiles may have a full complement of electrons if they also bear a full positive charge. Suggest an example that falls into this category. 

Friedel-Crafts acylation The reaction of an acyl halide, using stoichiometric amounts of a Lewis acid such as aluminium trihalide, with an aromatic compound to form the acylated derivative. This variation avoids the polysubstitution that commonly occurs in the Friedel-Crafts alkylation reaction. 

The reaction between alkyl (other than tertiary alkyl) bromides and phosphorus pentasulphide in the presence of aluminium trihalides yields mixtures of phos-phorodi- and phosphorotri-thioic bromides, (RS)P(S)Br2 and (RS)aP(S)Br other alkylating agents have previously been shown to give tetrathioesters. The 2,2-di-... 

The simple addition complexes of the aluminium trihalides are formally analogous to those of the boron trihalides. Phosphine and phosphorus trihalides form addition compounds of the type X3P-ADC3, where X = H or halogen and X = halogen (9.129). 

Alkylthiolate anions are more reactive by a factor of 10 —10 than OH towards tropylium cations and carbonyl compounds. Furthermore, PhSe is at least five times more nucleophilic than PhS , but the latter nucleophile, paradoxically, is capable of dealkylating PhSeMe and tertiary amines when in the presence of Pd, PdCl2, Pd(OAc)2, or RUCI3. Demethylation of methyl ethers and esters by treatment with a thiol in the presence of an aluminium trihalide has been reported. 

Hovel Organoboron-oxygen-halogenalumlnlum compounds from trlorganoboroxlnes with aluminium trihalides, R. Koster, K. Angermund, J. Serwatowskl and A. Sporzynskl, Cham. Ber.. 1986,... 

In some cases, in the reaction between metal carbonyls and the arene ligand, it is necessary to use aluminium trihalides as a catalyst, e.g. 

The imide [(MeInNC6F5)4] 5" was synthesized by a similar method. The tetrameric cubanes [In4X4(p-NBu )4] (where X Cl, Br or I) formed from the corresponding indium trihalide, InXa and LiHNBu have In4N4 heterocubane structures that are similar to their aluminium and gallium counterparts. The chloride [In4Cl4(NBu )4] is shown in Figure 8.11. 

Boron and aluminium give trihalides. BF3 is most conveniently made by heating BgO with concentrated H2SO4 and a fluoride. It reacts with aluminium chloride and bromide to produce involatile AIF3 and volatile BCI3. The volatility of the boron halides decreases with molecular weight. 

With the exception of thorium, the actinides form trihalides. For uranium and neptunium, reduction of the MX4 compounds with hydrogen is necessary, but for the elements from plutonium onwards the action of the carbon tetrahalide or aluminium halide on the dioxide is usually employed. The trifluorides are insoluble but the rest dissolve to give solutions containing ions. 

The well-known Sm ry -arene complexes are generally prepared by the reaction of arenes with rare-earth trihalides in the presence of aluminium halides. This reaction was also effective with samarium diiodide and was extended to thulium diiodide however, an attempt to react Tml2 with naphthalene in toluene in the presence of aluminium trichloride was not successful but resulted in the isolation of an ry -toluene complex of Tm [77 -(CH3C6H5)Tm(AlCl4)3] (Figure 6) (Fagin et al., 2005). 

At different times a variety of different reagents has been used for this purpose, among others phosphorus pentachloride [22,24,25], sulphuryl chloride [26], ferric chloride [26], aluminium chloride [24], stannic chloride [27,28], titanium tetrachloride [26] and boron trihalides [24,27,29]. Strong Lewis acids also react vigorously with the double bonds and the yields of tropylium salt are not good when they are used. 

A general method of preparation of alkyl phosphonous dihalides is the reaction of phosphorus trihalide with a dialkyl mercury, a dialkyl cadmium or a trialkyl aluminium. 

Complexes from the trihalides of aluminium and gallium with thiols, hydrogen sulphide and selenols, G.G. Hoffmann, Chem.Ber.. 1985, 118. 3320. 

Aluminium forms solid trihalides when it is heated with the halogens or hydrogen halides. Unlike the boron trihalides, these substances do not contain discrete MX3 molecules at room temperature. Thus, AIF3 melts at 1 290 °C and has a three-dimensional structure, characteristic of an ionic compound, in which each aluminium atom is surrounded by six fluorines, and each fluorine atom is nearly linearly coordinated to two aluminiums. AICI3, however, has a layer structure (Figure 9.20), which resembles that of MgCl2. 

Such effects, however, do not obliterate the strong resemblance of gallium, indium and thallium to aluminium that their presence in the same Group implies. All three elements are metals, which react with fluorine or chlorine to form trihalides, all of which are solids at room temperature. The metals also dissolve in dilute acids, evolving hydrogen and forming aqueous ions. With gallium and indium, these ions are Ga +(aq) and In (aq) we return to thallium in a moment. Addition of alkali precipitates colourless Ga(OH)3, which is amphoteric, and In(OH)3, which is not. 

With the exception of the metal and its alloys, aluminium occurs only in oxidation number +3 at normal temperatures the monohalides, formed by heating the trihalides with the metal, disproportionate on cooling. 

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