Question

The order of decreasing Lewis acid character as per online sources is as follows:

BCl3 > AlCl3 > GaCl3 > InCl3
Reason cited is: As we move down the group, the size of atom increases, and as a result, the tendency to attract electrons decreases. This leads to a decrease in Lewis acid nature down the group.
However, it is also known that BF3 is the weakest Lewis acid among the boron trihalides on account of back bonding effect (BI3 > BBr3 > BCl3 > BF3) as the energy gap between boron and fluorine (2p-2p) is less resulting in a stronger bond and the energy gap between boron and iodine (2p-5p) is the most, resulting in the weakest bond.
Why can't we apply the above explanation to BCl3, AlCl3, GaCl3, InCl3 as well, saying that Al and Cl should have the least energy gap (n=3 for both) and thus have the strongest bond and weakest Lewis acid character.
Please feel free to point out any fundamental mistakes in my assumptions and reasoning as I am still figuring things out and may have a limited/incorrect understanding of back bonding.

Answer 1

Lewis acid character of group 13 halides generally decreases down the group, with back bonding playing a significant role in BF3's reduced Lewis acidity. However, for AlCl3 and heavier analogs, the principle is less applicable due to the less efficient p-p overlap and more diffuse valence orbitals.

Step-by-step explanation:

The question suggests that the Lewis acid character of group 13 halides decreases down the group due to an increase in atomic size which reduces the ability to attract electrons. Conversely, it's noted that BF3 is less of a Lewis acid than other boron trihalides due to 'back bonding' where the relatively similar energy levels of B and F allow for better orbital overlap.

Your assessment raises the question of whether AlCl3 should similarly show less Lewis acid character due to a similar energy level between Al and Cl. However, this doesn't necessarily follow as the concept of back bonding is most pronounced with elements that can p–p overlap, such as B and F,

and less significant with heavier elements such as Al, Ga, and In whose empty orbitals are too large to effectively overlap with Cl's p electrons. Furthermore, as we move down the group, the valence orbitals become more diffused and less effective at forming such interactions.