Mg5H2(C4O9)2(CH3)8 crystallizes in the tetragonal I4_1/a space group. The structure consists of sixty-four 02329_fluka molecules inside a Mg5H2(C4O9)2 framework. In the Mg5H2(C4O9)2 framework, there are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(9) atom to form edge-sharing MgO6 octahedra. The Mg(1)-O(1) bond length is 2.10 Å. The Mg(1)-O(2) bond length is 2.16 Å. The Mg(1)-O(3) bond length is 2.10 Å. The Mg(1)-O(5) bond length is 2.03 Å. The Mg(1)-O(7) bond length is 2.15 Å. The Mg(1)-O(9) bond length is 2.03 Å. In the second Mg site, Mg(2) is bonded to two equivalent O(2), two equivalent O(6), and two equivalent O(7) atoms to form edge-sharing MgO6 octahedra. Both Mg(2)-O(2) bond lengths are 2.14 Å. Both Mg(2)-O(6) bond lengths are 2.04 Å. Both Mg(2)-O(7) bond lengths are 2.12 Å. In the third Mg site, Mg(3) is bonded to one O(1), one O(3), one O(4), one O(8), and two equivalent O(9) atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 58°. The Mg(3)-O(1) bond length is 2.25 Å. The Mg(3)-O(3) bond length is 2.14 Å. The Mg(3)-O(4) bond length is 2.03 Å. The Mg(3)-O(8) bond length is 2.08 Å. There is one shorter (2.01 Å) and one longer (2.03 Å) Mg(3)-O(9) bond length. There are four inequivalent C sites. In the first C site, C(5) is bonded in a distorted bent 120 degrees geometry to one O(5) and one O(6) atom. The C(5)-O(5) bond length is 1.27 Å. The C(5)-O(6) bond length is 1.27 Å. In the second C site, C(3) is bonded in a distorted bent 120 degrees geometry to one O(3) and one O(4) atom. The C(3)-O(3) bond length is 1.29 Å. The C(3)-O(4) bond length is 1.26 Å. In the third C site, C(7) is bonded in a distorted bent 120 degrees geometry to one O(7) and one O(8) atom. The C(7)-O(7) bond length is 1.30 Å. The C(7)-O(8) bond length is 1.26 Å. In the fourth C site, C(1) is bonded in a distorted bent 120 degrees geometry to one O(1) and one O(2) atom. The C(1)-O(1) bond length is 1.27 Å. The C(1)-O(2) bond length is 1.28 Å. H(1) is bonded in a single-bond geometry to one O(9) atom. The H(1)-O(9) bond length is 0.97 Å. There are nine inequivalent O sites. In the first O site, O(2) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(2), and one C(1) atom. In the second O site, O(6) is bonded in a distorted bent 120 degrees geometry to one Mg(2) and one C(5) atom. In the third O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(3), and one C(1) atom. In the fourth O site, O(5) is bonded in a bent 120 degrees geometry to one Mg(1) and one C(5) atom. In the fifth O site, O(9) is bonded in a distorted single-bond geometry to one Mg(1), two equivalent Mg(3), and one H(1) atom. In the sixth O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(3), and one C(3) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one C(7) atom. In the eighth O site, O(4) is bonded in a bent 150 degrees geometry to one Mg(3) and one C(3) atom. In the ninth O site, O(8) is bonded in a distorted bent 150 degrees geometry to one Mg(3) and one C(7) atom. Linkers: 32 CC([O])=O ,8 [OH]. Metal clusters: 20 [Mg]. The MOF has largest included sphere 4.26 A, density 1.25 g/cm3, surface area 4451.22 m2/g, accessible volume 0.33 cm3/g