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Which will have more lewis acid character out of $\ce{BCl3}$ and $\ce{AlCl3}$ ?

My attempt: In $\ce{BCl3}$, the back bonding takes place and there is 3pπ-2pπ overlap and in $\ce{AlCl3}$ it is 3pπ-3dπ overlap which is less effective than the first. So Al is more electron deficient as compared to B hence $\ce{AlCl3}$ must have higher lewis acid character but answer is its reverse. Please tell me where I'm wrong.

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Eventually, the same proposition applies. Boron, despite being more electron enriched by back bonding, can also accept other lone pairs more easily.

Most lone pair donors are from the 2nd or 3rd periods in the Periodic Table, as such the $\pi$ overlap with boron will be stronger than that in aluminium.

You should also notice that boron trichloride is not as substantially backbonded as boron trifluoride, hence the stability gained through back bonding is not strong enough. Because of this, the boron in the trichloride will still look out for more sources of electrons.

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  • $\begingroup$ Okay.. thanks for the help! But I was also thinking that AlCl3 dimerizes to Al2Cl6 and this does not happen in BCl3 that's why Boron is more electron deficient? $\endgroup$ – Varun Gupta Mar 8 at 5:57
  • $\begingroup$ That may account too. These data are often results of a multitude of factors, because of which it becomes difficult to predict which factors dominate and which don't. Also, thank you for reminding this. $\endgroup$ – Aabesh Ghosh Mar 8 at 18:30

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