The reactivity of the molecular magnesium hydride [Mg(Me(3)TACDAl(i)Bu(3))H] (1) featuring a terminal magnesiumhydrogen bond and an NNNN-type macrocyclic ligand, Me(3)TACD ((Me(3)TACD)H = Me-3[12]aneN(4) = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane), can be grouped into protonolysis, oxidation, hydrometalation, (insertion), and hydride abstraction. Protonolysis of 1 with weak Brnsted acids HX such as terminal acetylenes, amines, silanols, and silanes gave the corresponding derivatives [Mg(Me(3)TACDAl(i)Bu(3))X] (X = C=CPh, 3; HN(3,5-Me-2-C6H3), 4; OSiMe3, 5; OSiPh3, 6; Cl, 7; Br, 8). Single-crystal X-ray diffraction of anilide 4 showed a square-pyramidal coordination geometry for magnesium. No correlation with the pK(a) values of the acids was detected. Oxidation of 1 with elemental iodine gave the iodide [Mg(Me(3)TACDAl'Bu-3)I] (9), and oxidation with nitrous oxide afforded the mu-oxo-bridged compound [{Mg(Me(3)TACDAl(i)Bu3)}(2)(mu-O)] (10) with a linear MgOMg core, as characterized by single-crystal X-ray diffraction. The MgH bond reacted with benzaldehyde, benzophenone, fluorenone, and CO2 under insertion but not with the olefins 1,1,2-triphenylethylene, tert-butylethylene, and cyclopentene. The unstable formate, prepared also by salt metathesis of iodide 9 with potassium formate, revealed kappa O,kappa O' coordination in the solid state. Hydride abstraction with triphenylborane gave the ion pair [Mg(Me(3)TACDAl(i)Bu(3))(thf)]-[HBPh3] (16), which catalyzed the hydroboration of polar substrates by pinacolborane