An uncommon dehydrogenation mechanism for metal chloride-added ammonia borane (NH₃BH₃, AB) systems revealed that MgCl₂ interacts with the [NH₃] units in AB, analogous to the amine complex of Mg(NH₃)_xCl₂, thus not only resulting in a remarkable decrease in the hydrogen desorption temperature but also effectively suppressing undesirable volatile by-products, particularly ammonia gas.