MgH4(CO3)2 crystallizes in the orthorhombic Fddd space group. Mg(1) is bonded in an octahedral geometry to two equivalent O(2) and four equivalent O(1) atoms. Both Mg(1)-O(2) bond lengths are 2.05 Å. All Mg(1)-O(1) bond lengths are 2.10 Å. C(1) is bonded in a distorted bent 120 degrees geometry to two equivalent O(1) atoms. Both C(1)-O(1) bond lengths are 1.27 Å. H(1) is bonded in a single-bond geometry to one O(1) and one O(2) atom. The H(1)-O(1) bond length is 1.73 Å. The H(1)-O(2) bond length is 0.99 Å. There are two inequivalent O sites. In the first O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1) and two equivalent H(1) atoms. In the second O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one C(1), and one H(1) atom. Linkers: 2 [O]C(=O)C([O])=O. Metal clusters: 2 [Mg]. The MOF has largest included sphere 1.37 A, density 1.88 g/cm3, surface area 3840.50 m2/g, accessible volume 0.12 cm3/g