[yiyo]

I would like to ask which of the following is the strongest Lewis acid towards F-? (1)BF3 (2) SiF4 (3) SiCl4 (4) CF4  The answer is BF3....
--------------------------------------------------------------------------------
What factors should I consider in this question?? Should I consider the factor of hard acid and hard base? Or the octet rule? But if I consider the factor of hard acid and hard base, then BF3,SiF4 and SiCl4 are both hard.... And should I consider the fact that BF3 has a pi bond interaction between the p orbitals of B and F?? That fact makes me think that BF3 is a weaker acid than the others.....If I consider the octet rule, then BF3 seems to be the strongest Lewis acid.... As BF3 can become an octet when binded with F-, while the SiF4 and SiCl4 need to put the electrons in the d-orbitals, and CF4 is not possible as C can't have more than eight electrons..

[MrTeo]

I would like to ask which of the following is the strongest Lewis acid towards F-? (1)BF3 (2) SiF4 (3) SiCl4 (4) CF4  The answer is BF3...

Ok, you already started working on it and came out with a couple of good ideas. Anyway let's start from the begininning: which ones of these compounds can act as Lewis acids? (doing so we can already cross out one of the answers)

[yiyo]

I would like to ask which of the following is the strongest Lewis acid towards F-? (1)BF3 (2) SiF4 (3) SiCl4 (4) CF4  The answer is BF3...

Ok, you already started working on it and came out with a couple of good ideas. Anyway let's start from the begininning: which ones of these compounds can act as Lewis acids? (doing so we can already cross out one of the answers)

BF3, SiF4 and SiCl4 are Lewis acids? As they can accept electrons, but CF4 has 8 electrons already....

[MrTeo]

BF3, SiF4 and SiCl4 are Lewis acids? As they can accept electrons, but CF4 has 8 electrons already....

Right. Now, of the three species, the ones with Si can obviously accept a pair of electrons from F^- expanding their octet, but it won't be as advantageous as giving the same pair to BF₃, which, as you correctly pointed out before, doing so reaches the octet configuration. Another way to see this, which is anyway a little bit more "hands-on", is thinking about the fact that, even though they have the opportunity to accept more electrons, Si species won't be extremely good Lewis acids for their intrinsic electron abundance. On the other hand the lack of electrons of B, added to the presence of the fluorines and the absence of an octet configuration are all good reasons that make it a great Lewis acid (and a classic example of Lewis acid behaviour).