We propose two methods to facilitate the Mg–H bond cleavage of magnesium hydride MgH2. We found via density functional calculations that Mn (or Fe), when inserted between two MgH2, exhibits a much higher catalytic activity than the other 3d transition metal elements because of the induced spin polarization of the MgH2. We also found that an ionized MgH2 (MgH2+) has considerably softer Mg–H bonds. We can thus significantly reduce the H2 desorption temperatures from MgH2 to practical levels, ∼400K, by forming an alloy hydride with Mn (or Fe), and/or ionizing the MgH2.