MgCu2(NiO4)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 MgO6 octahedra that share corners with three equivalent Ni(1)O4 tetrahedra, corners with three equivalent Ni(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 Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form NiO4 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 36-51°. In the second Ni site, Ni(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form NiO4 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 57-62°. 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 Ni(1)O4 tetrahedra, corners with three equivalent Ni(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 eight inequivalent O sites. In the first O site, O(7) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Cu(1) atom. In the second O site, O(8) is bonded in a trigonal planar geometry to one Mg(1), one Ni(2), and one Cu(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Ni(1), and one Cu(1) atom. In the fourth O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Ni(1) and one Cu(2) atom. In the fifth O site, O(3) is bonded to two equivalent Mg(1), one Ni(2), and one Cu(1) atom to form distorted edge-sharing OMg2CuNi trigonal pyramids. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Cu(2) atom. In the seventh O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(2) and one Cu(1) atom. In the eighth O site, O(6) is bonded in a trigonal planar geometry to one Ni(2), one Cu(1), and one Cu(2) atom.
MgCu2(NiO4)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 MgO6 octahedra that share corners with three equivalent Ni(1)O4 tetrahedra, corners with three equivalent Ni(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 2.03 Å. The Mg(1)-O(4) bond length is 2.09 Å. The Mg(1)-O(7) bond length is 2.04 Å. The Mg(1)-O(8) bond length is 2.04 Å. There is one shorter (2.07 Å) and one longer (2.14 Å) Mg(1)-O(3) bond length. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(4), and one O(7) atom to form NiO4 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 36-51°. The Ni(1)-O(1) bond length is 1.85 Å. The Ni(1)-O(2) bond length is 1.83 Å. The Ni(1)-O(4) bond length is 1.84 Å. The Ni(1)-O(7) bond length is 1.79 Å. In the second Ni site, Ni(2) is bonded to one O(3), one O(5), one O(6), and one O(8) atom to form NiO4 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 57-62°. The Ni(2)-O(3) bond length is 1.84 Å. The Ni(2)-O(5) bond length is 1.83 Å. The Ni(2)-O(6) bond length is 1.88 Å. The Ni(2)-O(8) bond length is 1.83 Å. 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 Ni(1)O4 tetrahedra, corners with three equivalent Ni(2)O4 tetrahedra, and edges with two equivalent Mg(1)O6 octahedra. The Cu(1)-O(1) bond length is 1.83 Å. The Cu(1)-O(3) bond length is 1.99 Å. The Cu(1)-O(5) bond length is 1.81 Å. The Cu(1)-O(6) bond length is 1.91 Å. The Cu(1)-O(7) bond length is 2.08 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 1.84 Å. The Cu(2)-O(4) bond length is 1.85 Å. The Cu(2)-O(6) bond length is 1.89 Å. The Cu(2)-O(8) bond length is 1.84 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 Ni(1), and one Cu(1) atom. In the second O site, O(8) is bonded in a trigonal planar geometry to one Mg(1), one Ni(2), and one Cu(2) atom. In the third O site, O(1) is bonded in a trigonal planar geometry to one Mg(1), one Ni(1), and one Cu(1) atom. In the fourth O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Ni(1) and one Cu(2) atom. In the fifth O site, O(3) is bonded to two equivalent Mg(1), one Ni(2), and one Cu(1) atom to form distorted edge-sharing OMg2CuNi trigonal pyramids. In the sixth O site, O(4) is bonded in a distorted trigonal planar geometry to one Mg(1), one Ni(1), and one Cu(2) atom. In the seventh O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(2) and one Cu(1) atom. In the eighth O site, O(6) is bonded in a trigonal planar geometry to one Ni(2), one Cu(1), and one Cu(2) atom.