MgCu4(SO5)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent S(1)O4 tetrahedra and edges with two equivalent Cu(1)O5 trigonal bipyramids. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(2) atoms to form CuO5 trigonal bipyramids that share corners with three equivalent S(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Cu(1)O5 trigonal bipyramid. In the second Cu site, Cu(2) is bonded in a linear geometry to two equivalent O(2) atoms. S(1) is bonded to one O(3), one O(4), and two equivalent O(1) atoms to form SO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with three equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-56°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Cu(1), and one S(1) atom. In the second O site, O(2) is bonded to two equivalent Cu(1) and two equivalent Cu(2) atoms to form a mixture of corner and edge-sharing OCu4 tetrahedra. In the third O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Mg(1) and one S(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one Cu(1) and one S(1) atom.
MgCu4(SO5)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent S(1)O4 tetrahedra and edges with two equivalent Cu(1)O5 trigonal bipyramids. Both Mg(1)-O(3) bond lengths are 2.19 Å. All Mg(1)-O(1) bond lengths are 2.18 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(4), two equivalent O(1), and two equivalent O(2) atoms to form CuO5 trigonal bipyramids that share corners with three equivalent S(1)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Cu(1)O5 trigonal bipyramid. The Cu(1)-O(4) bond length is 1.99 Å. Both Cu(1)-O(1) bond lengths are 2.26 Å. There is one shorter (1.96 Å) and one longer (1.99 Å) Cu(1)-O(2) bond length. In the second Cu site, Cu(2) is bonded in a linear geometry to two equivalent O(2) atoms. Both Cu(2)-O(2) bond lengths are 1.82 Å. S(1) is bonded to one O(3), one O(4), and two equivalent O(1) atoms to form SO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with three equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-56°. The S(1)-O(3) bond length is 1.48 Å. The S(1)-O(4) bond length is 1.48 Å. Both S(1)-O(1) bond lengths are 1.52 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Mg(1), one Cu(1), and one S(1) atom. In the second O site, O(2) is bonded to two equivalent Cu(1) and two equivalent Cu(2) atoms to form a mixture of corner and edge-sharing OCu4 tetrahedra. In the third O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Mg(1) and one S(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 150 degrees geometry to one Cu(1) and one S(1) atom.