A Lewis acid is an electron-pair acceptor. BCl3, BeCl2, and BF3 are Lewis acids because their central atoms (Boron and Beryllium) have incomplete octets and can accept electron pairs. BaCl2 is an ionic compound formed between a metal (Ba) and a non-metal (Cl). While metal cations can sometimes act as Lewis acids by accepting electron density, Ba2+ in an ionic lattice or aqueous solution doesn't typically function as a Lewis acid in reactions with common Lewis bases in the way that the electron-deficient covalent boron/beryllium halides do.