Ca4MgB4H6(CO9)2 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(5), one O(6), and three equivalent O(4) atoms. Mg(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form MgO6 octahedra that share corners with two equivalent B(2)O4 tetrahedra, corners with four equivalent B(1)O4 tetrahedra, and edges with two equivalent B(2)O4 tetrahedra. There are two inequivalent B sites. In the first B site, B(1) is bonded to two equivalent O(2) and two equivalent O(3) atoms to form BO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with two equivalent B(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 69°. In the second B site, B(2) is bonded to one O(1), one O(5), and two equivalent O(2) atoms to form BO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent B(1)O4 tetrahedra, and  an edgeedge with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 63°. C(1) is bonded in a trigonal planar geometry to one O(6) and two equivalent O(4) atoms. There are two inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(3) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(5) atom. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ca(1), one Mg(1), and one B(2) atom to form distorted corner-sharing OCa2MgB tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(1), one Mg(1), one B(1), and one B(2) atom. In the third O site, O(3) is bonded in a distorted water-like geometry to one Ca(1), one B(1), and one H(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to three equivalent Ca(1) and one C(1) atom. In the fifth O site, O(5) is bonded in a distorted water-like geometry to two equivalent Ca(1), one B(2), and one H(2) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom.
Ca4MgB4H6(CO9)2 crystallizes in the monoclinic C2/m space group. Ca(1) is bonded in a 8-coordinate geometry to one O(1), one O(2), one O(3), one O(5), one O(6), and three equivalent O(4) atoms. The Ca(1)-O(1) bond length is 2.40 Å. The Ca(1)-O(2) bond length is 2.52 Å. The Ca(1)-O(3) bond length is 2.42 Å. The Ca(1)-O(5) bond length is 2.58 Å. The Ca(1)-O(6) bond length is 2.51 Å. There are a spread of Ca(1)-O(4) bond distances ranging from 2.45-2.99 Å. Mg(1) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form MgO6 octahedra that share corners with two equivalent B(2)O4 tetrahedra, corners with four equivalent B(1)O4 tetrahedra, and edges with two equivalent B(2)O4 tetrahedra. Both Mg(1)-O(1) bond lengths are 2.05 Å. All Mg(1)-O(2) bond lengths are 2.13 Å. There are two inequivalent B sites. In the first B site, B(1) is bonded to two equivalent O(2) and two equivalent O(3) atoms to form BO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with two equivalent B(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 69°. Both B(1)-O(2) bond lengths are 1.47 Å. Both B(1)-O(3) bond lengths are 1.49 Å. In the second B site, B(2) is bonded to one O(1), one O(5), and two equivalent O(2) atoms to form BO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent B(1)O4 tetrahedra, and  an edgeedge with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles are 63°. The B(2)-O(1) bond length is 1.41 Å. The B(2)-O(5) bond length is 1.51 Å. Both B(2)-O(2) bond lengths are 1.52 Å. C(1) is bonded in a trigonal planar geometry to one O(6) and two equivalent O(4) atoms. The C(1)-O(6) bond length is 1.30 Å. Both C(1)-O(4) bond lengths are 1.31 Å. There are two inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(3) atom. The H(1)-O(3) bond length is 0.97 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. There are six inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ca(1), one Mg(1), and one B(2) atom to form distorted corner-sharing OCa2MgB tetrahedra. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ca(1), one Mg(1), one B(1), and one B(2) atom. In the third O site, O(3) is bonded in a distorted water-like geometry to one Ca(1), one B(1), and one H(1) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to three equivalent Ca(1) and one C(1) atom. In the fifth O site, O(5) is bonded in a distorted water-like geometry to two equivalent Ca(1), one B(2), and one H(2) atom. In the sixth O site, O(6) is bonded in a distorted single-bond geometry to two equivalent Ca(1) and one C(1) atom.