M^III compounds

All aluminum halides can be made through direct reaction, but AlF₃ is best created through reaction with anhydrous HF. It has a structure that is based on corner-sharing AlF₆ octahedra (identical to ReO₃; see Topic D3). Solid AlCl₃ has a polymeric layer structure, but within the gas phase or nonpolar solvents is dimeric and molecular Al₂Cl₆.

The iodide and bromine comprise the molecular dimeric form in the solid state. The Aluminum halides are very strong Lewis acids and AlCl₃ is often used as an acid catalyst, for instance, in organic Friedel-Crafts reactions.

Complex halides consisting of the ions [AlF₆]^3- and [AlCl₄]^- are simply created and can be helpful for the preparation of compounds consisting of unusual cations like Cd₂²⁺.

The very stable form of the oxide Al₂O₃ is α-alumina with the corundum structure in which Al³⁺ ions take place 2/3 of the octahedral holes in a hexagonal close-packed oxide lattice. Other form γ-Al₂O₃ has a defect spinel structure. So-called β-alumina is actually a mixed oxide of aluminum of estimated formula NaAl₁₁O₁₇ with a disordered arrangement of Na⁺ ions and is a good ionic conductor.

Aluminum creates several mixed oxides of which the aluminosilicates are main constituents of minerals. Aluminum occasionally replaces a portion of the silicon exists as corner-sharing SiO₄ groups in these compounds. The mixed oxide mineral spinel MgAl₂O₄ provides its name to a significant structure type. 1/2 of the octahedral holes and one-eighth of the tetrahedral holes are filled within a cubic close-packed array of oxide ions. In the normal spinel form that is adopted through MgAl₂O₄ the divalent Mg²⁺ ion is in tetrahedral sites and the trivalent Al³⁺ is octahedral. A fraction of the cation sites are occupied at random in the defect spinel structure of γ-Al₂O₃.

Oxides and Halides of Ga and In are quite identical to those of Al, but contain less negative enthalpies of formation and (with In) a trend to higher coordination. Tl^III is more strongly oxidizing; for instance, there is no Tl^III iodide and the compound of stoichiometry Tl^I3 actually consists of Tl¹ with the linear tri-iodide ion I3 ^-.

Al, Ga and In create tetrahedrally coordinated solids with elements of group 15, that are part of the series of III-V semiconductors (that is groups 13-15, III-V in old nomenclature). The varied compounds gallium aluminum phosphide Ga_1-xAl_xP and the arsenide Ga_1-xAl_xAs are employed for light-emitting diode (LED) shows and semiconductor lasers.

Aluminum hydrides AlH3 comprise a structure identical to that of AlF₃. The tetra-hydroaluminate ion [AlH₄]^- is a very powerful reducing and hydride transfer agent, usually used in the form of lithium aluminum hydride' LiAlH₄ made through reaction of LiH with AlCl₃. Hydrides' stability decreases down the group but [GaH₄]^- is quite stable and the unstable digallane molecule Ga₂H₆ has been recognized with a structure such as that of diborane. Organoaluminum compounds are dimeric but the bonding is dissimilar from that of halides like the bridging methyl groups in Al₂(CH₃)₆ (1) must be held through three-center two-electron bonds identical to those in diborane. Organometallic compounds of Ga, In and Tl than for Al and do not dimerize are less stable.