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Which of the species $\ce{BF3}$ and $\ce{AlCl3}$ is more acidic, both are electron deficient.

In case of $\ce{BF3}$ one can argue that there is extensive p-π-p-π back bonding and as a result the electron deficiency is less.
That means that the positive charge on the overall molecule is less.

In case of $\ce{AlCl3}$ one can argue there is p-π-d-π back bonding and as a result the electron deficiency is more in case of aluminiuim.
Therefore the overall positive charge on the molecule is more, meaning it is more acidic.

So $\ce{AlCl3}$ is more acidic that $\ce{BF3}$ is conclusion of the above factors. Is this plausible?

I also faced the counter logic that $\ce{BF3}$ being a small compound tends to react more readily and therefore is a strong acid.

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  • $\begingroup$ Back bonding would dominate $\endgroup$ – Avnish Kabaj Jan 24 '18 at 8:15
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    $\begingroup$ D-orbitals in aluminium or chlorine? No. $\endgroup$ – Martin - マーチン Jan 24 '18 at 9:05
  • $\begingroup$ @Martin-マーチン why can you explain? $\endgroup$ – starunique2016 Jan 24 '18 at 9:38
  • $\begingroup$ [AlCl6]3- exists what about that $\endgroup$ – starunique2016 Jan 24 '18 at 9:40
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    $\begingroup$ @starunique2016 The molecular ion $\ce{[AlCl6]^3-}$ is predominantly bound by ionic interactions. The covalent part is best described with multi-centre bonds. In valence bond theory you have to invoke resonance. In none of these have the d-orbitals a role beyond polarisation effects. $\endgroup$ – Martin - マーチン Jan 24 '18 at 10:18
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$\ce{BF3}$ is less acidic than $\ce{AlCl3}$ as extent of octet completion is greater in $\ce{BF3}$ due to backbonding.

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  • $\begingroup$ No. There can be backbonding in Al-Cl systems. $\endgroup$ – Oscar Lanzi Jan 25 '18 at 2:38

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