Structure
"Aluminium tribromide" is really dialuminium hexabromide with the molecular formula of Al2Br6 in the solid state, solutions in noncoordinating solvents (e.g. CS2), and in the melt. Even upon evaporation, Al2Br6 exists in the gas phase. At high temperatures, the gaseous molecules break up into monomers:
- Al2Br6 → 2 AlBr3 ΔH°diss = 59 kJ/mol
Aluminium monobromide has been crystallographically characterized in the form the tetrameric adduct Al4Br4(NEt3)4 (Et = C2H5). This species is electronically related to cyclobutane. Theory suggest that the diatomic aluminium monobromide condenses to a dimer and then a tetrahedral cluster Al4Br4, akin to the analogous boron compound.
Al2Br6 consists of two AlBr4 tetrahedra that share a common edge. The molecular symmetry is D2h.
The monomer AlBr3, observed only in the vapor, can be described as trigonal planar, D3h point group. The atomic hybridization of aluminium is often described as sp2. The Br-Al-Br bond angles are 120 °.
Read more about this topic: Aluminium Bromide
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