MgU8Se17 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)MgU3 tetrahedra, corners with two equivalent Se(4)MgU3 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 37°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)MgU3 tetrahedra, corners with four equivalent Se(6)U4 trigonal pyramids, and faces with two equivalent Se(4)MgU3 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one Mg(1), one U(1), and two equivalent U(3) atoms to form distorted SeMgU3 tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra,  an edgeedge with one Se(4)MgU3 trigonal pyramid, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded to one Mg(1), one U(2), and two equivalent U(1) atoms to form distorted SeMgU3 trigonal pyramids that share corners with two equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with two equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  a faceface with one Se(7)U6 octahedra. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms.
MgU8Se17 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded in an octahedral geometry to two equivalent Se(4) and four equivalent Se(2) atoms. Both Mg(1)-Se(4) bond lengths are 2.54 Å. All Mg(1)-Se(2) bond lengths are 2.66 Å. There are three inequivalent U sites. In the first U site, U(1) is bonded in a 8-coordinate geometry to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), one Se(7), and two equivalent Se(1) atoms. The U(1)-Se(2) bond length is 2.93 Å. The U(1)-Se(3) bond length is 3.01 Å. The U(1)-Se(4) bond length is 2.90 Å. The U(1)-Se(5) bond length is 2.96 Å. The U(1)-Se(6) bond length is 3.00 Å. The U(1)-Se(7) bond length is 3.09 Å. There is one shorter (2.95 Å) and one longer (3.02 Å) U(1)-Se(1) bond length. In the second U site, U(2) is bonded in a 8-coordinate geometry to one Se(3), one Se(4), one Se(5), one Se(7), and four equivalent Se(1) atoms. The U(2)-Se(3) bond length is 2.89 Å. The U(2)-Se(4) bond length is 2.88 Å. The U(2)-Se(5) bond length is 2.96 Å. The U(2)-Se(7) bond length is 3.01 Å. There are two shorter (2.91 Å) and two longer (2.92 Å) U(2)-Se(1) bond lengths. In the third U site, U(3) is bonded in a 8-coordinate geometry to one Se(3), one Se(5), two equivalent Se(6), and four equivalent Se(2) atoms. The U(3)-Se(3) bond length is 3.00 Å. The U(3)-Se(5) bond length is 2.98 Å. There is one shorter (3.12 Å) and one longer (3.15 Å) U(3)-Se(6) bond length. There are two shorter (2.81 Å) and two longer (3.06 Å) U(3)-Se(2) bond lengths. There are seven inequivalent Se sites. In the first Se site, Se(6) is bonded to two equivalent U(1) and two equivalent U(3) atoms to form SeU4 trigonal pyramids that share corners with two equivalent Se(7)U6 octahedra, corners with six equivalent Se(2)MgU3 tetrahedra, corners with two equivalent Se(4)MgU3 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 37°. In the second Se site, Se(7) is bonded to two equivalent U(2) and four equivalent U(1) atoms to form SeU6 octahedra that share corners with four equivalent Se(2)MgU3 tetrahedra, corners with four equivalent Se(6)U4 trigonal pyramids, and faces with two equivalent Se(4)MgU3 trigonal pyramids. In the third Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent U(1) and two equivalent U(2) atoms. In the fourth Se site, Se(2) is bonded to one Mg(1), one U(1), and two equivalent U(3) atoms to form distorted SeMgU3 tetrahedra that share  a cornercorner with one Se(7)U6 octahedra, corners with five equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with three equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra,  an edgeedge with one Se(4)MgU3 trigonal pyramid, and  an edgeedge with one Se(6)U4 trigonal pyramid. The corner-sharing octahedral tilt angles are 53°. In the fifth Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms. In the sixth Se site, Se(4) is bonded to one Mg(1), one U(2), and two equivalent U(1) atoms to form distorted SeMgU3 trigonal pyramids that share corners with two equivalent Se(2)MgU3 tetrahedra,  a cornercorner with one Se(4)MgU3 trigonal pyramid, corners with two equivalent Se(6)U4 trigonal pyramids, edges with two equivalent Se(2)MgU3 tetrahedra, and  a faceface with one Se(7)U6 octahedra. In the seventh Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one U(2), one U(3), and two equivalent U(1) atoms.