MgCo4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 5-6°. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded in a rectangular see-saw-like geometry to one O(2), one O(3), and two equivalent O(1) atoms. In the second Co site, Co(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form CoO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(2)O6 octahedra. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form NiO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 5-6°. In the second Ni site, Ni(2) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form NiO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Co(1), one Co(2), one Ni(1), and one Ni(2) atom to form OMgCo2Ni2 trigonal bipyramids that share  a cornercorner with one O(2)MgCo3Ni square pyramid, corners with three equivalent O(3)Co3Ni tetrahedra, corners with five equivalent O(1)MgCo2Ni2 trigonal bipyramids, edges with three equivalent O(2)MgCo3Ni square pyramids,  an edgeedge with one O(3)Co3Ni tetrahedra, and edges with three equivalent O(1)MgCo2Ni2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mg(1), one Co(1), two equivalent Co(2), and one Ni(1) atom to form OMgCo3Ni square pyramids that share corners with four equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(3)Co3Ni tetrahedra, corners with two equivalent O(1)MgCo2Ni2 trigonal bipyramids,  an edgeedge with one O(3)Co3Ni tetrahedra, and edges with six equivalent O(1)MgCo2Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Co(1), two equivalent Co(2), and one Ni(2) atom to form OCo3Ni tetrahedra that share corners with three equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(3)Co3Ni tetrahedra, corners with six equivalent O(1)MgCo2Ni2 trigonal bipyramids,  an edgeedge with one O(2)MgCo3Ni square pyramid, and edges with two equivalent O(1)MgCo2Ni2 trigonal bipyramids.
MgCo4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 5-6°. Both Mg(1)-O(2) bond lengths are 2.07 Å. All Mg(1)-O(1) bond lengths are 2.12 Å. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded in a rectangular see-saw-like geometry to one O(2), one O(3), and two equivalent O(1) atoms. The Co(1)-O(2) bond length is 2.03 Å. The Co(1)-O(3) bond length is 1.88 Å. Both Co(1)-O(1) bond lengths are 2.06 Å. In the second Co site, Co(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form CoO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(2)O6 octahedra. Both Co(2)-O(1) bond lengths are 1.97 Å. Both Co(2)-O(2) bond lengths are 2.21 Å. Both Co(2)-O(3) bond lengths are 1.88 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form NiO6 octahedra that share corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 5-6°. Both Ni(1)-O(2) bond lengths are 2.11 Å. All Ni(1)-O(1) bond lengths are 2.18 Å. In the second Ni site, Ni(2) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form NiO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Co(2)O6 octahedra. Both Ni(2)-O(3) bond lengths are 1.97 Å. All Ni(2)-O(1) bond lengths are 2.12 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Co(1), one Co(2), one Ni(1), and one Ni(2) atom to form OMgCo2Ni2 trigonal bipyramids that share  a cornercorner with one O(2)MgCo3Ni square pyramid, corners with three equivalent O(3)Co3Ni tetrahedra, corners with five equivalent O(1)MgCo2Ni2 trigonal bipyramids, edges with three equivalent O(2)MgCo3Ni square pyramids,  an edgeedge with one O(3)Co3Ni tetrahedra, and edges with three equivalent O(1)MgCo2Ni2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mg(1), one Co(1), two equivalent Co(2), and one Ni(1) atom to form OMgCo3Ni square pyramids that share corners with four equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(3)Co3Ni tetrahedra, corners with two equivalent O(1)MgCo2Ni2 trigonal bipyramids,  an edgeedge with one O(3)Co3Ni tetrahedra, and edges with six equivalent O(1)MgCo2Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Co(1), two equivalent Co(2), and one Ni(2) atom to form OCo3Ni tetrahedra that share corners with three equivalent O(2)MgCo3Ni square pyramids, corners with three equivalent O(3)Co3Ni tetrahedra, corners with six equivalent O(1)MgCo2Ni2 trigonal bipyramids,  an edgeedge with one O(2)MgCo3Ni square pyramid, and edges with two equivalent O(1)MgCo2Ni2 trigonal bipyramids.