Which of the following halogens can act as a mild Lewis acid?

Bromine and iodine can both act as mild Lewis acids due to their ability to accept electron pairs. The concept of a Lewis acid revolves around the ability to accept electrons to form a coordinate covalent bond. While these halogens are generally better known for their roles as electron donors in various contexts, they do possess vacant orbitals that can facilitate Lewis acidic behavior.

Bromine, in particular, can engage in acceptance of electron density due to its relatively larger size compared to fluorine and chlorine, allowing for better orbital overlap when interacting with electron-rich species. Iodine, being even larger than bromine, can also exhibit Lewis acidity but is less effective because of its diffuse electron cloud and decreased electronegativity.

Fluorine, on the other hand, is highly electronegative and primarily acts as a Lewis base rather than as a Lewis acid. Its small size and full outer shell make it less favorable for accepting electrons compared to its larger counterparts. Thus, both bromine and iodine fit the criteria for mild Lewis acids, making the conclusion that they can collectively act as such completely valid.