Mg(CuO2)4 crystallizes in the trigonal R-3m space group. Mg(1) is bonded to six equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Cu(1)O6 octahedra and edges with six equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to six equivalent O(1) atoms to form CuO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra and edges with six equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. In the second Cu site, Cu(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form CuO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Cu(2)O6 octahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cu(1), and two equivalent Cu(2) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to three equivalent Cu(2) atoms.
Mg(CuO2)4 crystallizes in the trigonal R-3m space group. Mg(1) is bonded to six equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Cu(1)O6 octahedra and edges with six equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. All Mg(1)-O(1) bond lengths are 2.12 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to six equivalent O(1) atoms to form CuO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra and edges with six equivalent Cu(2)O6 octahedra. The corner-sharing octahedral tilt angles are 10°. All Cu(1)-O(1) bond lengths are 2.05 Å. In the second Cu site, Cu(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form CuO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Cu(1)O6 octahedra, and edges with four equivalent Cu(2)O6 octahedra. Both Cu(2)-O(2) bond lengths are 1.97 Å. All Cu(2)-O(1) bond lengths are 1.95 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Cu(1), and two equivalent Cu(2) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to three equivalent Cu(2) atoms.