MgCu2Tl(SeO5)2 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(3), one O(7), one O(8), and two equivalent O(6) atoms to form MgO5 trigonal bipyramids that share  a cornercorner with one Se(1)O4 tetrahedra and corners with two equivalent Se(2)O4 tetrahedra. Cu(1) is bonded in a distorted rectangular see-saw-like geometry to one O(5), one O(6), one O(7), and one O(8) atom. Tl(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(5) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(3), and two equivalent O(5) atoms to form SeO4 tetrahedra that share  a cornercorner with one Mg(1)O5 trigonal bipyramid. In the second Se site, Se(2) is bonded to one O(2), one O(4), and two equivalent O(6) atoms to form SeO4 tetrahedra that share corners with two equivalent Mg(1)O5 trigonal bipyramids. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Tl(1) and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Tl(1) and one Se(2) atom. In the third O site, O(3) is bonded in a linear geometry to one Mg(1) and one Se(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Cu(1), one Tl(1), and one Se(1) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Mg(1), one Cu(1), and one Se(2) atom. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Mg(1) and two equivalent Cu(1) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Mg(1) and two equivalent Cu(1) atoms.
MgCu2Tl(SeO5)2 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(3), one O(7), one O(8), and two equivalent O(6) atoms to form MgO5 trigonal bipyramids that share  a cornercorner with one Se(1)O4 tetrahedra and corners with two equivalent Se(2)O4 tetrahedra. The Mg(1)-O(3) bond length is 2.38 Å. The Mg(1)-O(7) bond length is 2.04 Å. The Mg(1)-O(8) bond length is 1.96 Å. Both Mg(1)-O(6) bond lengths are 2.13 Å. Cu(1) is bonded in a distorted rectangular see-saw-like geometry to one O(5), one O(6), one O(7), and one O(8) atom. The Cu(1)-O(5) bond length is 1.95 Å. The Cu(1)-O(6) bond length is 1.99 Å. The Cu(1)-O(7) bond length is 1.85 Å. The Cu(1)-O(8) bond length is 1.87 Å. Tl(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(5) atoms. The Tl(1)-O(1) bond length is 2.73 Å. The Tl(1)-O(2) bond length is 2.84 Å. Both Tl(1)-O(5) bond lengths are 2.67 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(3), and two equivalent O(5) atoms to form SeO4 tetrahedra that share  a cornercorner with one Mg(1)O5 trigonal bipyramid. The Se(1)-O(1) bond length is 1.66 Å. The Se(1)-O(3) bond length is 1.67 Å. Both Se(1)-O(5) bond lengths are 1.71 Å. In the second Se site, Se(2) is bonded to one O(2), one O(4), and two equivalent O(6) atoms to form SeO4 tetrahedra that share corners with two equivalent Mg(1)O5 trigonal bipyramids. The Se(2)-O(2) bond length is 1.67 Å. The Se(2)-O(4) bond length is 1.63 Å. Both Se(2)-O(6) bond lengths are 1.73 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Tl(1) and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Tl(1) and one Se(2) atom. In the third O site, O(3) is bonded in a linear geometry to one Mg(1) and one Se(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Cu(1), one Tl(1), and one Se(1) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Mg(1), one Cu(1), and one Se(2) atom. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Mg(1) and two equivalent Cu(1) atoms. In the eighth O site, O(8) is bonded in a trigonal planar geometry to one Mg(1) and two equivalent Cu(1) atoms.