H₃BO₃ is

The boron atom in boric acid, H₃BO₃ is electron deficient, thereby it acts as a Lewis acid with one vacant p-orbital. There is no d-orbital in boron atom, hence it can accommodate only one electron pair in its vacant p-orbital. This implies that boric acid is a weak Lewis acid.

Boric acid is a monobasic acid which undergoes ionization with water and releases proton from it.

${\mathrm{H}}_3{\mathrm{BO}}_3+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{B}{\left(\mathrm{OH}\right)}_4^{-}+{\mathrm{H}}^{+}$