ErH4(CO2)4 crystallizes in the monoclinic P2_1 space group. Er(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom. The Er(1)-O(1) bond length is 2.38 Å. The Er(1)-O(2) bond length is 2.29 Å. The Er(1)-O(3) bond length is 2.27 Å. The Er(1)-O(4) bond length is 2.30 Å. The Er(1)-O(5) bond length is 2.31 Å. The Er(1)-O(6) bond length is 2.37 Å. The Er(1)-O(7) bond length is 2.55 Å. The Er(1)-O(8) bond length is 2.45 Å. There are four 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 1.24 Å. In the second C site, C(2) is bonded in a trigonal planar geometry to one H(2), one O(3), and one O(4) atom. The C(2)-H(2) bond length is 0.93 Å. The C(2)-O(3) bond length is 1.19 Å. The C(2)-O(4) bond length is 1.18 Å. In the third C site, C(3) is bonded in a trigonal planar geometry to one H(3), one O(5), and one O(6) atom. The C(3)-H(3) bond length is 0.93 Å. The C(3)-O(5) bond length is 1.23 Å. The C(3)-O(6) bond length is 1.18 Å. In the fourth C site, C(4) is bonded in a trigonal planar geometry to one H(4), one O(7), and one O(8) atom. The C(4)-H(4) bond length is 0.93 Å. The C(4)-O(7) bond length is 1.26 Å. The C(4)-O(8) bond length is 1.25 Å. There are four 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 single-bond geometry to one C(4) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Er(1) and one C(1) atom. In the third O site, O(3) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(2) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one Er(1) and one C(2) atom. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Er(1) and one C(3) atom. In the sixth O site, O(6) is bonded in a 2-coordinate geometry to one Er(1) and one C(3) atom. In the seventh O site, O(7) is bonded in a single-bond geometry to one Er(1) and one C(4) atom. In the eighth O site, O(8) is bonded in a distorted single-bond geometry to one Er(1) and one C(4) atom. Linkers: 7 [O]C=O. Metal clusters: 2 [Er]. The MOF has largest included sphere 4.11 A, density 2.13 g/cm3, surface area 2605.35 m2/g, accessible volume 0.15 cm3/g