Which of the following is not a Lewis acid ?

A Lewis acid is characterized by its ability to accept at least one lone pair using an empty orbital of right energy. AlCl₃, SnCl₄, and FeCl₃ act as Lewis acids because of the ability of their central atoms to accept lone pairs in their empty orbitals. AlCl₃.6H₂O or [Al(H₂O)₆]Cl₃ cannot act as a Lewis acid because the central ion Al³⁺ is no longer in a position to accept the lone pair as its coordination number is fulfilled.