The strongest Lewis acid among boron halides is (RPMT 2010)

According to lewis acid base theory, lewis acid means tendency to accept a pair of electron. In the above case only halogen atom is changing, means if the halogen atom or halide is more electronegative then the repulsions will increase which will make the molecule more difficult to accept a lone pair of electron and thus making it less acidic. Order of electronegativity is as follows: F < Cl < Br < I Back bonding between halogen and boron decreases as the size of p-orbital of halogen increases. The increasing order of size of p-orbital in halogens F < Cl < Br < I The order of lewis acidity is: