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).