The reaction of a manganese(V)-oxo porphyrinoid complex with the Lewis acid B(C6F5)(3) leads to reversible stabilization of the valence tautomer Mn-IV(O)(pi-radical cation). The latter complex, in combination with B(C6F5)(3), reacts with ArO-H substrates via formal hydrogen-atom transfer and exhibits dramatically increased reaction rates over the Mn-IV(O) starting material.