CaC5N2H4O5 crystallizes in the monoclinic P2_1/c space group. Ca(1) is bonded to one N(1), one O(2), one O(3), one O(4), one O(5), and two equivalent O(1) atoms to form corner-sharing CaNO6 pentagonal bipyramids. The Ca(1)-N(1) bond length is 2.52 Å. The Ca(1)-O(2) bond length is 2.40 Å. The Ca(1)-O(3) bond length is 2.38 Å. The Ca(1)-O(4) bond length is 2.38 Å. The Ca(1)-O(5) bond length is 2.38 Å. There is one shorter (2.46 Å) and one longer (2.55 Å) Ca(1)-O(1) bond length. There are five inequivalent C sites. In the first C site, C(4) is bonded in a distorted single-bond geometry to one C(3), one C(5), and one N(2) atom. The C(4)-C(3) bond length is 1.39 Å. The C(4)-C(5) bond length is 1.49 Å. The C(4)-N(2) bond length is 1.36 Å. In the second C site, C(5) is bonded in a distorted bent 120 degrees geometry to one C(4), one O(3), and one O(4) atom. The C(5)-O(3) bond length is 1.27 Å. The C(5)-O(4) bond length is 1.28 Å. In the third C site, C(1) is bonded in a distorted bent 120 degrees geometry to one C(2), one O(1), and one O(2) atom. The C(1)-C(2) bond length is 1.47 Å. The C(1)-O(1) bond length is 1.28 Å. The C(1)-O(2) bond length is 1.27 Å. In the fourth C site, C(2) is bonded in a distorted single-bond geometry to one C(1), one C(3), and one N(1) atom. The C(2)-C(3) bond length is 1.40 Å. The C(2)-N(1) bond length is 1.35 Å. In the fifth C site, C(3) is bonded in a distorted single-bond geometry to one C(2), one C(4), and one H(4) atom. The C(3)-H(4) bond length is 1.08 Å. There are two inequivalent N sites. In the first N site, N(1) is bonded in a 3-coordinate geometry to one Ca(1), one C(2), and one N(2) atom. The N(1)-N(2) bond length is 1.33 Å. In the second N site, N(2) is bonded in a 2-coordinate geometry to one C(4), one N(1), and one H(3) atom. The N(2)-H(3) bond length is 1.04 Å. There are four inequivalent H sites. In the first H site, H(1) is bonded in a distorted single-bond geometry to one O(2) and one O(5) atom. The H(1)-O(2) bond length is 1.69 Å. The H(1)-O(5) bond length is 1.00 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(5) atom. The H(2)-O(5) bond length is 0.98 Å. In the third H site, H(3) is bonded in a distorted single-bond geometry to one N(2) and one O(4) atom. The H(3)-O(4) bond length is 1.70 Å. In the fourth H site, H(4) is bonded in a single-bond geometry to one C(3) atom. There are five inequivalent O sites. In the first O site, O(5) is bonded in a distorted water-like geometry to one Ca(1), one H(1), and one H(2) atom. In the second O site, O(4) is bonded in a 3-coordinate geometry to one Ca(1), one C(5), and one H(3) atom. In the third O site, O(1) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom. In the fourth O site, O(2) is bonded in a distorted single-bond geometry to one Ca(1), one C(1), and one H(1) atom. In the fifth O site, O(3) is bonded in a 2-coordinate geometry to one Ca(1) and one C(5) atom. Linkers: 4 [O]C(=O)c1cc(C([O])=O)[nH]n1. Metal clusters: 4 [Ca]. The MOF has largest included sphere 1.57 A, density 1.97 g/cm3, surface area 3228.51 m2/g, accessible volume 0.07 cm3/g