Er2H9(C2O5)4 crystallizes in the monoclinic C2/c space group. Er(1) is bonded in a 10-coordinate geometry to one O(1), one O(10), one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), one O(8), and one O(9) atom. The Er(1)-O(1) bond length is 2.36 Å. The Er(1)-O(10) bond length is 2.40 Å. The Er(1)-O(2) bond length is 2.34 Å. The Er(1)-O(3) bond length is 2.26 Å. The Er(1)-O(4) bond length is 2.35 Å. The Er(1)-O(5) bond length is 2.33 Å. The Er(1)-O(6) bond length is 2.29 Å. The Er(1)-O(7) bond length is 2.41 Å. The Er(1)-O(8) bond length is 2.30 Å. The Er(1)-O(9) bond length is 2.51 Å. There are five inequivalent C sites. In the first C site, 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 1.22 Å. The C(1)-O(2) bond length is 1.27 Å. In the second C site, C(2) is bonded in a trigonal planar geometry to one H(2), one O(5), and one O(6) atom. The C(2)-H(2) bond length is 0.93 Å. The C(2)-O(5) bond length is 1.22 Å. The C(2)-O(6) bond length is 1.21 Å. In the third C site, C(3) is bonded in a 4-coordinate geometry to one H(3), one O(4), one O(7), and one O(9) atom. The C(3)-H(3) bond length is 0.93 Å. The C(3)-O(4) bond length is 1.22 Å. The C(3)-O(7) bond length is 1.21 Å. The C(3)-O(9) bond length is 1.21 Å. In the fourth C site, C(4) is bonded in a trigonal planar geometry to one H(5) and two equivalent O(3) atoms. The C(4)-H(5) bond length is 0.92 Å. Both C(4)-O(3) bond lengths are 1.20 Å. In the fifth C site, C(5) is bonded in a hexagonal planar geometry to two equivalent H(4), two equivalent O(10), and two equivalent O(8) atoms. Both C(5)-H(4) bond lengths are 0.93 Å. Both C(5)-O(10) bond lengths are 1.21 Å. Both C(5)-O(8) bond lengths are 1.21 Å. There are five inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one C(1) atom. In the second H site, H(2) is bonded in a single-bond geometry to one C(2) atom. In the third H site, H(3) is bonded in a single-bond geometry to one C(3) atom. In the fourth H site, H(4) is bonded in a distorted T-shaped geometry to one C(5), one O(10), and one O(8) atom. The H(4)-O(10) bond length is 1.04 Å. The H(4)-O(8) bond length is 1.05 Å. In the fifth H site, H(5) is bonded in a single-bond geometry to one C(4) atom. There are ten inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 120 degrees geometry to one Er(1) and one C(1) atom. In the second O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(1) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Er(1) and one C(4) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Er(1), one C(3), and one O(9) atom. The O(4)-O(9) bond length is 0.97 Å. In the fifth O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(2) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Er(1) and one C(2) atom. In the seventh O site, O(7) is bonded in a distorted L-shaped geometry to one Er(1) and one C(3) atom. In the eighth O site, O(8) is bonded in a 4-coordinate geometry to one Er(1), one C(5), one H(4), and one O(10) atom. The O(8)-O(10) bond length is 1.34 Å. In the ninth O site, O(9) is bonded in a 2-coordinate geometry to one Er(1), one C(3), and one O(4) atom. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to one Er(1), one C(5), one H(4), and one O(8) atom. Linkers: 4 [O]C1OO1 ,1 [O]O[CH2]=O.[O] ,1 [O]OCO[O] ,2 O=[CH2]=O.[O].[O]. Metal clusters: 8 [Er]. The MOF has largest included sphere 4.27 A, density 2.16 g/cm3, surface area 2398.63 m2/g, accessible volume 0.23 cm3/g