Lewis acids are those compounds which have incomplete octate or complete octate which is able to keep more electrons in its outer shell.
act as Lewis acids as they can extend their coordination number beyond 4 due to the presence of d−orbitals.
So they are able to keep greater than 8 electrons in its outermost orbit. Thus, increase its coordination number and they work as a Lewis acid.
can not keep more than 8 electrons in its outermost orbit due to the absence of d-orbitals. So, it not act as lewis acid.