MgH3(CO2)3 crystallizes in the trigonal R-3 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in an octahedral geometry to six equivalent O(1) atoms. All Mg(1)-O(1) bond lengths are 2.08 Å. In the second Mg site, Mg(2) is bonded in an octahedral geometry to six equivalent O(2) atoms. All Mg(2)-O(2) bond lengths are 2.08 Å. C(1) is bonded in a trigonal planar geometry to one H(1), one O(1), and one O(2) atom. The C(1)-H(1) bond length is 0.93 Å. The C(1)-O(1) bond length is 1.23 Å. The C(1)-O(2) bond length is 1.21 Å. H(1) is bonded in a single-bond geometry to one C(1) atom. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Mg(1) and one C(1) atom. In the second O site, O(2) is bonded in a bent 120 degrees geometry to one Mg(2) and one C(1) atom. Linkers: 18 [O]C=O. Metal clusters: 6 [Mg]. The MOF has largest included sphere 4.13 A, density 1.20 g/cm3, surface area 4669.82 m2/g, accessible volume 0.36 cm3/g