In both the molecules: BF3 or BBr3, boron has only O six electrons in its outermost shell and can accept two electrons to complete its octet. Hence, both are Lewis ex acids. In BBr3, size of overlapping orbitals i.e. 2p orbital ha of boron and 4p orbital of bromine differ. Hence, for overlapping does not occur to a great extent and as a result electron deficiency of boron does not decrease to a great extent. But in BF, size of 2p orbital of boron and 2p orbital of fluorine is almost same. Hencе, рë-рë bonding occurs effectively (Back bonding) which decreases electron density on boron. Thus, BBr3 is he stronger Lewis acid than BF3.
