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  • Would the acidity of the aluminium trihalides be the same as that for the boron trihalides?

The interesting thing about the acidity of the boron trihalides is that they actually increase in acidity from fluorine to iodine. This is due to the presence of stronger pi bonding in boron trifluoride compared to the other boron trihalides, which means that stronger partial pi bonds would have to be broken when boron accepts an electron pair from a base.

  • Would we observe this trend in acidity in the aluminium trihalides as well?
  • Based on the same reasoning, we should be able to right?
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I have chanced upon the answer in an inorganic chemistry textbook (Essential Trends in Inorganic Chemistry by D. M. P. Mingos) I was reading. It turns out that this trend only holds for the boron trihalides and does not apply to the aluminium trihalides. However, the book did not provide the explanation for the exception of boron.

My thinking is that perhaps, backbonding is not present in the aluminium trihalides, possibly due to incompatibility of the p atomic orbitals for pi bonding between aluminium and the halogen atoms. Does anyone else have any insights to share?

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  • $\begingroup$ Aluminium trichloride typically exists as a dimer or as a polymer (or so I've read). This would suggest that the orbitals to too "rigid(?)" to bond properly, and intermolecular bonding is preferred. $\endgroup$
    – Eashaan Godbole
    Commented Oct 20, 2017 at 15:41

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