Ba2C4H2O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 10-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(7), two equivalent O(6), and two equivalent O(8) atoms. The Ba(1)-O(1) bond length is 2.79 Å. The Ba(1)-O(2) bond length is 2.85 Å. The Ba(1)-O(3) bond length is 2.83 Å. The Ba(1)-O(4) bond length is 2.81 Å. The Ba(1)-O(5) bond length is 3.01 Å. The Ba(1)-O(7) bond length is 3.20 Å. There is one shorter (2.76 Å) and one longer (2.89 Å) Ba(1)-O(6) bond length. There is one shorter (2.82 Å) and one longer (3.03 Å) Ba(1)-O(8) bond length. In the second Ba site, Ba(2) is bonded in a 9-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(9), two equivalent O(5), and two equivalent O(7) atoms. The Ba(2)-O(1) bond length is 2.80 Å. The Ba(2)-O(2) bond length is 2.90 Å. The Ba(2)-O(3) bond length is 2.91 Å. The Ba(2)-O(4) bond length is 2.81 Å. The Ba(2)-O(9) bond length is 2.83 Å. There is one shorter (2.74 Å) and one longer (2.84 Å) Ba(2)-O(5) bond length. There is one shorter (2.83 Å) and one longer (2.86 Å) Ba(2)-O(7) bond length. There are four inequivalent C sites. In the first C site, C(1) is bonded in a distorted bent 120 degrees geometry to one O(1) and one O(2) atom. The C(1)-O(1) bond length is 1.26 Å. The C(1)-O(2) bond length is 1.27 Å. In the second C site, C(2) is bonded in a distorted bent 120 degrees geometry to one O(3) and one O(4) atom. The C(2)-O(3) bond length is 1.27 Å. The C(2)-O(4) bond length is 1.27 Å. In the third C site, C(3) is bonded in a distorted bent 120 degrees geometry to one O(5) and one O(6) atom. The C(3)-O(5) bond length is 1.28 Å. The C(3)-O(6) bond length is 1.26 Å. In the fourth C site, C(4) is bonded in a distorted bent 120 degrees geometry to one O(7) and one O(8) atom. The C(4)-O(7) bond length is 1.26 Å. The C(4)-O(8) bond length is 1.28 Å. There are two inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(9) atom. The H(1)-O(9) bond length is 0.99 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(9) atom. The H(2)-O(9) bond length is 0.97 Å. There are nine inequivalent O sites. In the first O site, O(9) is bonded in a water-like geometry to one Ba(2), one H(1), and one H(2) atom. In the second O site, O(1) is bonded in a distorted single-bond geometry to one Ba(1), one Ba(2), and one C(1) atom. In the third O site, O(2) is bonded in a distorted single-bond geometry to one Ba(1), one Ba(2), and one C(1) atom. In the fourth O site, O(3) is bonded in a distorted single-bond geometry to one Ba(1), one Ba(2), and one C(2) atom. In the fifth O site, O(4) is bonded in a distorted single-bond geometry to one Ba(1), one Ba(2), and one C(2) atom. In the sixth O site, O(5) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one C(3) atom. In the seventh O site, O(6) is bonded in a distorted single-bond geometry to two equivalent Ba(1) and one C(3) atom. In the eighth O site, O(7) is bonded in a distorted single-bond geometry to one Ba(1), two equivalent Ba(2), and one C(4) atom. In the ninth O site, O(8) is bonded in a single-bond geometry to two equivalent Ba(1) and one C(4) atom. Linkers: 4 [O]C(=O)C([O])=O. Metal clusters: 4 [Ba]. The MOF has largest included sphere 1.66 A, density 3.40 g/cm3, surface area 1762.36 m2/g, accessible volume 0.04 cm3/g