MgCo4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Co(3)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-7°. There are three inequivalent Co sites. In the first Co site, Co(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1), one O(3), and two equivalent O(2) atoms. In the second Co site, Co(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form CoO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(3)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. In the third Co site, Co(3) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form CoO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-7°. Ni(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NiO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Co(3)O6 octahedra, and edges with two equivalent Ni(1)O6 octahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Co(1), one Co(2), and two equivalent Ni(1) atoms to form OCo2Ni2 tetrahedra that share corners with three equivalent O(3)MgCo2Ni2 square pyramids, corners with six equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra,  an edgeedge with one O(3)MgCo2Ni2 square pyramid, and edges with two equivalent O(2)MgCo3Ni square pyramids. In the second O site, O(2) is bonded to one Mg(1), one Co(1), one Co(2), one Co(3), and one Ni(1) atom to form OMgCo3Ni square pyramids that share  a cornercorner with one O(3)MgCo2Ni2 square pyramid, corners with five equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra, edges with three equivalent O(3)MgCo2Ni2 square pyramids, edges with three equivalent O(2)MgCo3Ni square pyramids, and  an edgeedge with one O(1)Co2Ni2 tetrahedra. In the third O site, O(3) is bonded to one Mg(1), one Co(1), one Co(3), and two equivalent Ni(1) atoms to form distorted OMgCo2Ni2 square pyramids that share corners with two equivalent O(2)MgCo3Ni square pyramids, corners with four equivalent O(3)MgCo2Ni2 square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra, edges with six equivalent O(2)MgCo3Ni square pyramids, and  an edgeedge with one O(1)Co2Ni2 tetrahedra.
MgCo4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Co(3)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-7°. Both Mg(1)-O(3) bond lengths are 2.23 Å. All Mg(1)-O(2) bond lengths are 2.09 Å. There are three inequivalent Co sites. In the first Co site, Co(1) is bonded in a distorted rectangular see-saw-like geometry to one O(1), one O(3), and two equivalent O(2) atoms. The Co(1)-O(1) bond length is 1.84 Å. The Co(1)-O(3) bond length is 2.02 Å. Both Co(1)-O(2) bond lengths are 2.10 Å. In the second Co site, Co(2) is bonded to two equivalent O(1) and four equivalent O(2) atoms to form CoO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(3)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. Both Co(2)-O(1) bond lengths are 1.88 Å. All Co(2)-O(2) bond lengths are 2.02 Å. In the third Co site, Co(3) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form CoO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-7°. Both Co(3)-O(3) bond lengths are 1.94 Å. All Co(3)-O(2) bond lengths are 2.21 Å. Ni(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form NiO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Co(3)O6 octahedra, and edges with two equivalent Ni(1)O6 octahedra. Both Ni(1)-O(1) bond lengths are 1.94 Å. Both Ni(1)-O(2) bond lengths are 2.13 Å. Both Ni(1)-O(3) bond lengths are 2.15 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Co(1), one Co(2), and two equivalent Ni(1) atoms to form OCo2Ni2 tetrahedra that share corners with three equivalent O(3)MgCo2Ni2 square pyramids, corners with six equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra,  an edgeedge with one O(3)MgCo2Ni2 square pyramid, and edges with two equivalent O(2)MgCo3Ni square pyramids. In the second O site, O(2) is bonded to one Mg(1), one Co(1), one Co(2), one Co(3), and one Ni(1) atom to form OMgCo3Ni square pyramids that share  a cornercorner with one O(3)MgCo2Ni2 square pyramid, corners with five equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra, edges with three equivalent O(3)MgCo2Ni2 square pyramids, edges with three equivalent O(2)MgCo3Ni square pyramids, and  an edgeedge with one O(1)Co2Ni2 tetrahedra. In the third O site, O(3) is bonded to one Mg(1), one Co(1), one Co(3), and two equivalent Ni(1) atoms to form distorted OMgCo2Ni2 square pyramids that share corners with two equivalent O(2)MgCo3Ni square pyramids, corners with four equivalent O(3)MgCo2Ni2 square pyramids, corners with three equivalent O(1)Co2Ni2 tetrahedra, edges with six equivalent O(2)MgCo3Ni square pyramids, and  an edgeedge with one O(1)Co2Ni2 tetrahedra.