Covalent structures and bonding

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Anhydrous aluminium chloride would be expected to be ionically bonded, but it exists under certain conditions as a covalently bonded dimer. What is the reason for this behaviour?

Corrie writes .....
 
Al³⁺ ions have a very high charge density on account of their relatively small size and high charge. Chloride ions are relatviely large. So, if AlCl₃ were ionic, the Al³⁺ ions would so distort/polarise the outer electrons on the chloride ion that the bonding develops a large amount of covalent character. In fact, to such an extent that AlCl₃ in solution in some solvents and in the vapour state exists as a dimer molecule, Al₂Cl₆, as you have noted.
 
AlCl₃ as a covalent molecule is electron deficient. The outer shell of the Al atom only has 6 electrons. By forming a dimer, in which a lone pair on one of Cl atoms in an AlCl₃ molecule forms a dative bond to the Al atom in another AlCl₃ molecule, and vice versa, Al can complete its outer shell.

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updated: 20 December 2007