The strengths of boron halides as Lewis acids are in the order ${\text{BI}}_3>{\text{BBr}}_3>{\text{BCl}}_3>{\text{BF}}_3$ . This order is the opposite of what is expected on steric grounds as well as from considerations of electronegatively. It can be explained in terms of boron-halogen-bonding.
Here, acidic nature $\propto \frac{1}{\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{t }\mathrm{}\mathrm{o}\mathrm{f }\mathrm{}\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{b}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{g}}$