page_title: Compounds of aluminium
section_title: Inorganic compounds
hierarchical_section_title: Compounds of aluminium / Inorganic compounds
caption_attribution_description: English: mechanism of the Friedel-Crafts acylation reaction
context_page_description: Aluminium combines characteristics of pre- and post-transition metals. Since it has few available electrons for metallic bonding, like its heavier group 13 congeners, it has the characteristic physical properties of a post-transition metal, with longer-than-expected interatomic distances. Furthermore, as Al³⁺ is a small and highly charged cation, it is strongly polarizing and aluminium compounds tend towards covalency; this behaviour is similar to that of beryllium, an example of a diagonal relationship. However, unlike all other post-transition metals, the underlying core under aluminium's valence shell is that of the preceding noble gas, whereas for gallium and indium it is that of the preceding noble gas plus a filled d-subshell, and for thallium and nihonium it is that of the preceding noble gas plus filled d- and f-subshells. Hence, aluminium does not suffer the effects of incomplete shielding of valence electrons by inner electrons from the nucleus that its heavier congeners do.
context_section_description: The vast majority of compounds, including all aluminium-containing minerals and all commercially significant aluminium compounds, feature aluminium in the oxidation state 3+. The coordination number of such compounds varies, but generally Al³⁺ is either six- or four-coordinate. Almost all compounds of aluminium(III) are colorless.
  In aqueous solution, Al³⁺ exists as the hexaaqua cation [Al(H₂O)₆]³⁺, which has an approximate pKₐ of 10⁻⁵. Such solutions are acidic as this cation can act as a proton donor, progressively hydrolysing to [Al(H₂O)₅(OH)]²⁺, [Al(H₂O)₄(OH)₂]⁺, and so on. As pH increases these mononuclear species begin to aggregate together by the formation of hydroxide bridges, forming many oligomeric ions, such as the Keggin ion [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺. The process ends with precipitation of aluminium hydroxide, Al(OH)₃. This is useful for clarification of water, as the precipitate nucleates on suspended particles in the water, hence removing them. Increasing the pH even further leads to the hydroxide dissolving again as aluminate, [Al(H₂O)₂(OH)₄]⁻, is formed. Aluminium hydroxide forms both salts and aluminates and dissolves in acid and alkali, as well as on fusion with acidic and basic oxides:
  Al₂O₃ + 3 SiO₂  Al₂(SiO₃)₃
  Al₂O₃ + CaO  Ca(AlO₂)₂
  This behaviour of Al(OH)₃ is termed amphoterism, and is characteristic of weakly basic cations that form insoluble hydroxides and whose hydrated species can also donate their protons. Further examples include Be²⁺, Zn²⁺, Ga³⁺, Sn²⁺, and Pb²⁺; indeed, gallium in the same group is slightly more acidic than aluminium. One effect of this is that aluminium salts with weak acids are hydrolysed in water to the aquated hydroxide and the corresponding nonmetal hydride: aluminium sulfide yields hydrogen sulfide, aluminium nitride yields ammonia, and aluminium carbide yields methane. Aluminium cyanide, acetate, and carbonate exist in aqueous solution but are unstable as such; only incomplete hydrolysis takes place for salts with strong acids, such as the halides, nitrate, and sulfate. For similar reasons, anhydrous aluminium salts cannot be made by heating their "hydrates": hydrated aluminium chloride is in fact not AlCl₃·6H₂O but [Al(H₂O)₆]Cl₃, and the Al–O bonds are so strong that heating is not sufficient to break them and form Al–Cl bonds instead:
  2[Al(H₂O)₆]Cl₃  Al₂O₃ + 6 HCl + 9 H₂O
  All four trihalides are well known. Unlike the structures of the three heavier trihalides, aluminium fluoride (AlF₃) features six-coordinate aluminium, which explains its involatility and insolubility as well as high heat of formation. Each aluminium atom is surrounded by six fluorine atoms in a distorted octahedral arrangement, with each fluorine atom being shared between the corners of two octahedra in a structure related to but distorted from that of ReO₃. Such {AlF₆} units also exist in complex fluorides such as cryolite, Na₃AlF₆, but should not be considered as [AlF₆]³⁻ complex anions as the Al–F bonds are not significantly different in type from the other M–F bonds. Such differences in coordination between the fluorides and heavier halides are not unusual, occurring in Snᴵⱽ and Biᴵᴵᴵ as well for example; even bigger differences occur between CO₂ and SiO₂. AlF₃ melts at 1,290 °C (2,354 °F) and is made by reaction of aluminium oxide with hydrogen fluoride gas at 700 °C (1,292 °F).
  With heavier halides, the coordination numbers are lower. The other trihalides are dimeric or polymeric with tetrahedral four-coordinate aluminium centers. Aluminium trichloride (AlCl₃) has a layered polymeric structure below its melting point of 192.4 °C (378 °F), but transforms on melting to Al₂Cl₆ dimers with a concomitant increase in volume by 85% and a near-total loss of electrical conductivity. These still predominate in the gas phase at low temperatures (150–200 °C), but at higher temperatures increasingly dissociate into trigonal planar AlCl₃ monomers similar to the structure of BCl₃. Aluminium tribromide and aluminium triiodide form Al₂X₆ dimers in all three phases and hence do not show such significant changes
caption: Mechanism of the Friedel–Crafts acylation, using AlCl3 as a catalyst
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