MgCu2(SbO4)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(4), one O(7), one O(8), and two equivalent O(3) atoms to form distorted MgO6 octahedra that share corners with three equivalent Sb(1)O4 tetrahedra, corners with three equivalent Sb(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, 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(1), one O(3), one O(5), one O(6), and one O(7) atom to form CuO5 trigonal bipyramids that share corners with two equivalent Sb(1)O4 tetrahedra, corners with three equivalent Sb(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. In the second Cu site, Cu(2) is bonded in a distorted rectangular see-saw-like geometry to one O(2), one O(4), one O(6), and one O(8) atom. There are two inequivalent Sb sites. In the first Sb site, Sb(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form SbO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with two equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 36-52°. In the second Sb site, Sb(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form SbO4 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 57-61°. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cu(1), and one Sb(1) atom. In the second O site, O(8) is bonded in a trigonal planar geometry to one Mg(1), one Cu(2), and one Sb(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Cu(1), and one Sb(1) atom. In the fourth O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Cu(2) and one Sb(1) atom. In the fifth O site, O(3) is bonded to two equivalent Mg(1), one Cu(1), and one Sb(2) atom to form distorted edge-sharing OMg2CuSb trigonal pyramids. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cu(2), and one Sb(1) atom. In the seventh O site, O(5) is bonded in a bent 120 degrees geometry to one Cu(1) and one Sb(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Cu(1), one Cu(2), and one Sb(2) atom.
MgCu2(SbO4)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(4), one O(7), one O(8), and two equivalent O(3) atoms to form distorted MgO6 octahedra that share corners with three equivalent Sb(1)O4 tetrahedra, corners with three equivalent Sb(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Cu(1)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 2.07 Å. The Mg(1)-O(4) bond length is 2.12 Å. The Mg(1)-O(7) bond length is 2.04 Å. The Mg(1)-O(8) bond length is 2.06 Å. There is one shorter (2.11 Å) and one longer (2.36 Å) Mg(1)-O(3) bond length. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form CuO5 trigonal bipyramids that share corners with two equivalent Sb(1)O4 tetrahedra, corners with three equivalent Sb(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. The Cu(1)-O(1) bond length is 1.93 Å. The Cu(1)-O(3) bond length is 2.13 Å. The Cu(1)-O(5) bond length is 1.89 Å. The Cu(1)-O(6) bond length is 2.03 Å. The Cu(1)-O(7) bond length is 2.19 Å. In the second Cu site, Cu(2) is bonded in a distorted rectangular see-saw-like geometry to one O(2), one O(4), one O(6), and one O(8) atom. The Cu(2)-O(2) bond length is 1.91 Å. The Cu(2)-O(4) bond length is 1.94 Å. The Cu(2)-O(6) bond length is 2.04 Å. The Cu(2)-O(8) bond length is 1.97 Å. There are two inequivalent Sb sites. In the first Sb site, Sb(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form SbO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with two equivalent Cu(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 36-52°. The Sb(1)-O(1) bond length is 1.92 Å. The Sb(1)-O(2) bond length is 1.90 Å. The Sb(1)-O(4) bond length is 1.93 Å. The Sb(1)-O(7) bond length is 1.89 Å. In the second Sb site, Sb(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form SbO4 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 57-61°. The Sb(2)-O(3) bond length is 1.94 Å. The Sb(2)-O(5) bond length is 1.90 Å. The Sb(2)-O(6) bond length is 1.97 Å. The Sb(2)-O(8) bond length is 1.90 Å. There are eight inequivalent O sites. In the first O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cu(1), and one Sb(1) atom. In the second O site, O(8) is bonded in a trigonal planar geometry to one Mg(1), one Cu(2), and one Sb(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Cu(1), and one Sb(1) atom. In the fourth O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Cu(2) and one Sb(1) atom. In the fifth O site, O(3) is bonded to two equivalent Mg(1), one Cu(1), and one Sb(2) atom to form distorted edge-sharing OMg2CuSb trigonal pyramids. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cu(2), and one Sb(1) atom. In the seventh O site, O(5) is bonded in a bent 120 degrees geometry to one Cu(1) and one Sb(2) atom. In the eighth O site, O(6) is bonded in a distorted trigonal planar geometry to one Cu(1), one Cu(2), and one Sb(2) atom.