The ammonia complex of magnesium borohydride Mg(BH4)(2)center dot 2NH(3) (I), which contains 16.0 wt% hydrogen, is a potentially promising material for hydrogen storage. This complex was synthesized by thermal decomposition of a hexaaammine complex Mg(BH4)(2)center dot 6NH(3) (II), which crystallizes in the cubic space group Fm (3) over barm with unit cell parameter a = 10.82(1) angstrom and is isostructural to Mg(NH3)6Cl(2). We solved the structure of I that crystallizes in the orthorhombic space group Pcab with unit cell parameters a = 17.4872(4) angstrom, b = 9.4132(2) angstrom, c = 8.7304(2) angstrom, and Z = 8. This structure is built from individual pseudotetrahedral molecules Mg(BH4)(2)center dot 2NH(3) containing one bidentate BH4 group and one tridentate BH4 group that pack into a layered crystal structure mediated by N-H center dot center dot center dot H-B dihydrogen bonds. Complex I decomposes endothermically starting at 150 degrees C, with a maximum hydrogen release rate at 205 degrees C, which makes it competitive with ammonia borane BH3NH3 as a hydrogen storage material.