Mg2CO5 crystallizes in the trigonal R3c space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(2), one O(4), one O(5), and two equivalent O(1) atoms to form distorted MgO5 square pyramids that share corners with two equivalent Mg(2)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, and  an edgeedge with one Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 35-56°. The Mg(1)-O(2) bond length is 2.20 Å. The Mg(1)-O(4) bond length is 2.00 Å. The Mg(1)-O(5) bond length is 2.28 Å. There is one shorter (2.01 Å) and one longer (2.15 Å) Mg(1)-O(1) bond length. In the second Mg site, Mg(2) is bonded to one O(2), one O(5), two equivalent O(3), and two equivalent O(4) atoms to form MgO6 octahedra that share corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Mg(2)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The Mg(2)-O(2) bond length is 2.07 Å. The Mg(2)-O(5) bond length is 2.19 Å. There is one shorter (2.09 Å) and one longer (2.15 Å) Mg(2)-O(3) bond length. There is one shorter (2.05 Å) and one longer (2.14 Å) Mg(2)-O(4) bond length. C(1) is bonded in a trigonal planar geometry to one O(1), one O(2), and one O(3) atom. The C(1)-O(1) bond length is 1.27 Å. The C(1)-O(2) bond length is 1.30 Å. The C(1)-O(3) bond length is 1.29 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Mg(1) and one C(1) atom. In the second 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 third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Mg(2) and one C(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Mg(1) and two equivalent Mg(2) atoms. In the fifth O site, O(5) is bonded in an L-shaped geometry to one Mg(1) and one Mg(2) atom. Linkers: 18 [O]C([O])=O. Metal clusters: 36 [Mg]. The MOF has largest included sphere 8.32 A, density 1.25 g/cm3, surface area 2954.50 m2/g, accessible volume 0.43 cm3/g