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AlCl3 is said to be an electron deficient molecule as Al only has 6 electrons in its valence shell. Why doesn’t it just form a dative bond with one of the chlorine’s? I know this gives the more electronegative chlorine a positive formal charge, but then the diner Al2Cl6 shouldn’t exist either.

How does forming a dative bond with a chlorine from a different molecule instead of its own make the situation more stable?

Vulgar Mechanick
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  • It does - https://www.chemguide.co.uk/atoms/bonding/dative.html – Waylander Jan 18 '21 at 22:26
  • You misunderstood my question. I’m talking about the dative bond coming from the same molecule – Vulgar Mechanick Jan 18 '21 at 22:42
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    If you wonder why dimerisation of AlCl3 is exothermic then the wording of your post isn't particularly good. – Mithoron Jan 19 '21 at 00:09
  • I’m not asking if the dimerisation itself is exothermic. I’m asking why Al doesn’t form a dative bond with one of its own Chlroines, when not in a diner state, to complete its octet – Vulgar Mechanick Jan 19 '21 at 00:14
  • :/ AlCl3 has 3 partially double bonds. – Mithoron Jan 19 '21 at 01:16
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    They kind of do. Martin's answer here https://chemistry.stackexchange.com/a/64600/16683 shows that although the pi backbonding is not as strong as in BCl3, there is some degree of pi backbonding from Cl to Al in the monomer. You need to think in terms of "somewhere between one and two bonds". The followup question would then be why it prefers to dimerise rather than stay on its own, but that's addressed by the linked question. – orthocresol Jan 19 '21 at 01:38

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