MgCd2P2H4(CO3)4 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to two equivalent O(2), two equivalent O(5), and two equivalent O(6) atoms to form MgO6 octahedra that share corners with four equivalent P(1)CO3 tetrahedra. Both Mg(1)-O(2) bond lengths are 2.17 Å. Both Mg(1)-O(5) bond lengths are 2.14 Å. Both Mg(1)-O(6) bond lengths are 1.97 Å. Cd(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms. The Cd(1)-O(1) bond length is 2.40 Å. The Cd(1)-O(2) bond length is 2.32 Å. The Cd(1)-O(3) bond length is 2.35 Å. The Cd(1)-O(5) bond length is 2.33 Å. There is one shorter (2.26 Å) and one longer (2.35 Å) Cd(1)-O(4) bond length. There are two inequivalent C sites. In the first C site, C(1) is bonded in a distorted bent 120 degrees geometry to one C(2), one O(2), and one O(3) atom. The C(1)-C(2) bond length is 1.53 Å. The C(1)-O(2) bond length is 1.27 Å. The C(1)-O(3) bond length is 1.27 Å. In the second C site, C(2) is bonded in a 4-coordinate geometry to one C(1), one P(1), one H(2), and one O(1) atom. The C(2)-P(1) bond length is 1.86 Å. The C(2)-H(2) bond length is 1.11 Å. The C(2)-O(1) bond length is 1.42 Å. P(1) is bonded to one C(2), one O(4), one O(5), and one O(6) atom to form PCO3 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 18-48°. The P(1)-O(4) bond length is 1.56 Å. The P(1)-O(5) bond length is 1.55 Å. The P(1)-O(6) bond length is 1.50 Å. There are two inequivalent H sites. In the first H site, H(1) is bonded in a distorted linear geometry to one O(1) and one O(3) atom. The H(1)-O(1) bond length is 1.01 Å. The H(1)-O(3) bond length is 1.62 Å. In the second H site, H(2) is bonded in a single-bond geometry to one C(2) atom. There are six inequivalent O sites. In the first O site, O(3) is bonded in a distorted trigonal planar geometry to one Cd(1), one C(1), and one H(1) atom. In the second O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Cd(1) and one P(1) atom. In the third O site, O(5) is bonded in a 3-coordinate geometry to one Mg(1), one Cd(1), and one P(1) atom. In the fourth O site, O(6) is bonded in a bent 150 degrees geometry to one Mg(1) and one P(1) atom. In the fifth O site, O(1) is bonded in a 2-coordinate geometry to one Cd(1), one C(2), and one H(1) atom. In the sixth O site, O(2) is bonded in a 3-coordinate geometry to one Mg(1), one Cd(1), and one C(1) atom. Linkers: 2 [O]C(=O)[C@H](O)P([O])([O])=O ,2 [O]C(=O)[C@@H](O)P([O])([O])=O. Metal clusters: 2 [Mg] ,4 [Cd]. The MOF has largest included sphere 1.73 A, density 3.34 g/cm3, surface area 1771.43 m2/g, accessible volume 0.05 cm3/g