Eu6Mg7H26 crystallizes in the monoclinic C2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one H(8), one H(9), two equivalent H(1), and two equivalent H(2) atoms to form distorted corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 30-50°. In the second Mg site, Mg(2) is bonded to one H(5), one H(6), two equivalent H(3), and two equivalent H(4) atoms to form corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 24-60°. In the third Mg site, Mg(3) is bonded to two equivalent H(4), two equivalent H(7), and two equivalent H(8) atoms to form a mixture of edge and corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 50-60°. In the fourth Mg site, Mg(4) is bonded to two equivalent H(9) and four equivalent H(3) atoms to form corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 24-30°. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 10-coordinate geometry to one H(7), one H(9), two equivalent H(4), two equivalent H(5), and four equivalent H(1) atoms. In the second Eu site, Eu(2) is bonded in a 11-coordinate geometry to two equivalent H(3), two equivalent H(4), three equivalent H(6), and four equivalent H(2) atoms. In the third Eu site, Eu(3) is bonded in a 7-coordinate geometry to one H(5), two equivalent H(1), two equivalent H(2), and two equivalent H(3) atoms. There are nine inequivalent H sites. In the first H site, H(3) is bonded in a 4-coordinate geometry to one Mg(2), one Mg(4), one Eu(2), and one Eu(3) atom. In the second H site, H(4) is bonded to one Mg(2), one Mg(3), one Eu(1), and one Eu(2) atom to form distorted HEu2Mg2 tetrahedra that share corners with two equivalent H(4)Eu2Mg2 tetrahedra, corners with four equivalent H(1)Eu3Mg tetrahedra, corners with four equivalent H(2)Eu3Mg tetrahedra, and  an edgeedge with one H(4)Eu2Mg2 tetrahedra. In the third H site, H(5) is bonded in a 2-coordinate geometry to one Mg(2), one Eu(3), and two equivalent Eu(1) atoms. In the fourth H site, H(6) is bonded in a distorted single-bond geometry to one Mg(2) and three equivalent Eu(2) atoms. In the fifth H site, H(7) is bonded in a distorted L-shaped geometry to two equivalent Mg(3) and one Eu(1) atom. In the sixth H site, H(8) is bonded in a distorted trigonal planar geometry to one Mg(1) and two equivalent Mg(3) atoms. In the seventh H site, H(9) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(4), and one Eu(1) atom. In the eighth H site, H(1) is bonded to one Mg(1), one Eu(3), and two equivalent Eu(1) atoms to form distorted HEu3Mg tetrahedra that share corners with two equivalent H(1)Eu3Mg tetrahedra, corners with two equivalent H(2)Eu3Mg tetrahedra, corners with four equivalent H(4)Eu2Mg2 tetrahedra,  an edgeedge with one H(2)Eu3Mg tetrahedra, and edges with three equivalent H(1)Eu3Mg tetrahedra. In the ninth H site, H(2) is bonded to one Mg(1), one Eu(3), and two equivalent Eu(2) atoms to form distorted HEu3Mg tetrahedra that share corners with two equivalent H(1)Eu3Mg tetrahedra, corners with two equivalent H(2)Eu3Mg tetrahedra, corners with four equivalent H(4)Eu2Mg2 tetrahedra,  an edgeedge with one H(1)Eu3Mg tetrahedra, and edges with three equivalent H(2)Eu3Mg tetrahedra.
Eu6Mg7H26 crystallizes in the monoclinic C2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one H(8), one H(9), two equivalent H(1), and two equivalent H(2) atoms to form distorted corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 30-50°. The Mg(1)-H(8) bond length is 1.88 Å. The Mg(1)-H(9) bond length is 2.13 Å. Both Mg(1)-H(1) bond lengths are 1.93 Å. Both Mg(1)-H(2) bond lengths are 1.94 Å. In the second Mg site, Mg(2) is bonded to one H(5), one H(6), two equivalent H(3), and two equivalent H(4) atoms to form corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 24-60°. The Mg(2)-H(5) bond length is 1.88 Å. The Mg(2)-H(6) bond length is 1.93 Å. Both Mg(2)-H(3) bond lengths are 2.00 Å. Both Mg(2)-H(4) bond lengths are 1.90 Å. In the third Mg site, Mg(3) is bonded to two equivalent H(4), two equivalent H(7), and two equivalent H(8) atoms to form a mixture of edge and corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 50-60°. Both Mg(3)-H(4) bond lengths are 1.93 Å. Both Mg(3)-H(7) bond lengths are 1.92 Å. Both Mg(3)-H(8) bond lengths are 1.89 Å. In the fourth Mg site, Mg(4) is bonded to two equivalent H(9) and four equivalent H(3) atoms to form corner-sharing MgH6 octahedra. The corner-sharing octahedral tilt angles range from 24-30°. Both Mg(4)-H(9) bond lengths are 1.90 Å. All Mg(4)-H(3) bond lengths are 1.94 Å. There are three inequivalent Eu sites. In the first Eu site, Eu(1) is bonded in a 10-coordinate geometry to one H(7), one H(9), two equivalent H(4), two equivalent H(5), and four equivalent H(1) atoms. The Eu(1)-H(7) bond length is 2.58 Å. The Eu(1)-H(9) bond length is 2.45 Å. Both Eu(1)-H(4) bond lengths are 2.61 Å. Both Eu(1)-H(5) bond lengths are 2.86 Å. There are two shorter (2.37 Å) and two longer (2.51 Å) Eu(1)-H(1) bond lengths. In the second Eu site, Eu(2) is bonded in a 11-coordinate geometry to two equivalent H(3), two equivalent H(4), three equivalent H(6), and four equivalent H(2) atoms. Both Eu(2)-H(3) bond lengths are 2.78 Å. Both Eu(2)-H(4) bond lengths are 2.56 Å. There is one shorter (2.61 Å) and two longer (2.78 Å) Eu(2)-H(6) bond lengths. There are two shorter (2.46 Å) and two longer (2.50 Å) Eu(2)-H(2) bond lengths. In the third Eu site, Eu(3) is bonded in a 7-coordinate geometry to one H(5), two equivalent H(1), two equivalent H(2), and two equivalent H(3) atoms. The Eu(3)-H(5) bond length is 2.37 Å. Both Eu(3)-H(1) bond lengths are 2.48 Å. Both Eu(3)-H(2) bond lengths are 2.35 Å. Both Eu(3)-H(3) bond lengths are 2.47 Å. There are nine inequivalent H sites. In the first H site, H(3) is bonded in a 4-coordinate geometry to one Mg(2), one Mg(4), one Eu(2), and one Eu(3) atom. In the second H site, H(4) is bonded to one Mg(2), one Mg(3), one Eu(1), and one Eu(2) atom to form distorted HEu2Mg2 tetrahedra that share corners with two equivalent H(4)Eu2Mg2 tetrahedra, corners with four equivalent H(1)Eu3Mg tetrahedra, corners with four equivalent H(2)Eu3Mg tetrahedra, and  an edgeedge with one H(4)Eu2Mg2 tetrahedra. In the third H site, H(5) is bonded in a 2-coordinate geometry to one Mg(2), one Eu(3), and two equivalent Eu(1) atoms. In the fourth H site, H(6) is bonded in a distorted single-bond geometry to one Mg(2) and three equivalent Eu(2) atoms. In the fifth H site, H(7) is bonded in a distorted L-shaped geometry to two equivalent Mg(3) and one Eu(1) atom. In the sixth H site, H(8) is bonded in a distorted trigonal planar geometry to one Mg(1) and two equivalent Mg(3) atoms. In the seventh H site, H(9) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(4), and one Eu(1) atom. In the eighth H site, H(1) is bonded to one Mg(1), one Eu(3), and two equivalent Eu(1) atoms to form distorted HEu3Mg tetrahedra that share corners with two equivalent H(1)Eu3Mg tetrahedra, corners with two equivalent H(2)Eu3Mg tetrahedra, corners with four equivalent H(4)Eu2Mg2 tetrahedra,  an edgeedge with one H(2)Eu3Mg tetrahedra, and edges with three equivalent H(1)Eu3Mg tetrahedra. In the ninth H site, H(2) is bonded to one Mg(1), one Eu(3), and two equivalent Eu(2) atoms to form distorted HEu3Mg tetrahedra that share corners with two equivalent H(1)Eu3Mg tetrahedra, corners with two equivalent H(2)Eu3Mg tetrahedra, corners with four equivalent H(4)Eu2Mg2 tetrahedra,  an edgeedge with one H(1)Eu3Mg tetrahedra, and edges with three equivalent H(2)Eu3Mg tetrahedra.