ErRu2P4H8(C2O7)2 crystallizes in the monoclinic C2/c space group. Er(1) is bonded in a 4-coordinate geometry to two equivalent O(3) and two equivalent O(6) atoms. Both Er(1)-O(3) bond lengths are 2.33 Å. Both Er(1)-O(6) bond lengths are 2.28 Å. Ru(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1), one O(2), one O(4), and one O(5) atom. The Ru(1)-O(1) bond length is 2.01 Å. The Ru(1)-O(2) bond length is 2.03 Å. The Ru(1)-O(4) bond length is 2.02 Å. The Ru(1)-O(5) bond length is 2.03 Å. There are two inequivalent C sites. In the first C site, C(1) is bonded in a distorted tetrahedral geometry to one C(2), one P(1), one P(2), and one O(7) atom. The C(1)-C(2) bond length is 1.46 Å. The C(1)-P(1) bond length is 1.85 Å. The C(1)-P(2) bond length is 1.85 Å. The C(1)-O(7) bond length is 1.41 Å. In the second C site, C(2) is bonded in a trigonal non-coplanar geometry to one C(1); one H(4); and two equivalent H(2,3) atoms. The C(2)-H(4) bond length is 0.98 Å. Both C(2)-H(2,3) bond lengths are 0.98 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one C(1), one O(1), one O(2), and one O(3) atom to form corner-sharing PCO3 tetrahedra. The P(1)-O(1) bond length is 1.54 Å. The P(1)-O(2) bond length is 1.54 Å. The P(1)-O(3) bond length is 1.50 Å. In the second P site, P(2) is bonded to one C(1), one O(4), one O(5), and one O(6) atom to form corner-sharing PCO3 tetrahedra. The P(2)-O(4) bond length is 1.54 Å. The P(2)-O(5) bond length is 1.55 Å. The P(2)-O(6) bond length is 1.49 Å. There are three inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(7) atom. The H(1)-O(7) bond length is 0.84 Å. In the second H site, H(2,3) is bonded in a single-bond geometry to one C(2) atom. In the third H site, H(4) is bonded in a single-bond geometry to one C(2) atom. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Ru(1) and one P(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Ru(1) and one P(1) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Ru(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ru(1) and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one P(2) atom. In the seventh O site, O(7) is bonded in a water-like geometry to one C(1) and one H(1) atom. Linkers: 4 CC(O)(P([O])([O])=O)P([O])([O])=O. Metal clusters: 2 [Er] ,4 [Ru]. The MOF has largest included sphere 4.81 A, density 1.86 g/cm3, surface area 2319.91 m2/g, accessible volume 0.31 cm3/g