MgH3(CO2)3 crystallizes in the orthorhombic Pnna space group. Mg(1) is bonded in an octahedral geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. Both Mg(1)-O(1) bond lengths are 2.09 Å. Both Mg(1)-O(2) bond lengths are 2.05 Å. Both Mg(1)-O(3) bond lengths are 2.07 Å. There are two inequivalent C sites. In the first C site, C(1) is bonded in a trigonal planar geometry to one H(1), one O(2), and one O(3) atom. The C(1)-H(1) bond length is 0.95 Å. The C(1)-O(2) bond length is 1.24 Å. The C(1)-O(3) bond length is 1.26 Å. In the second C site, C(2) is bonded in a trigonal planar geometry to one H(2) and two equivalent O(1) atoms. The C(2)-H(2) bond length is 0.95 Å. Both C(2)-O(1) bond lengths are 1.26 Å. There are two inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one C(1) atom. In the second H site, H(2) is bonded in a single-bond geometry to one C(2) atom. There are three inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one Mg(1) and one C(2) atom. In the second O site, O(2) is bonded in a bent 120 degrees geometry to one Mg(1) and one C(1) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Mg(1) and one C(1) atom. Linkers: 12 [O]C=O. Metal clusters: 4 [Mg]. The MOF has largest included sphere 3.84 A, density 1.27 g/cm3, surface area 4498.80 m2/g, accessible volume 0.32 cm3/g