MgCo3NiO8 is beta indium sulfide-derived structured and crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one O(3), one O(6), and two equivalent O(5) atoms. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form CoO6 octahedra that share edges with two equivalent Co(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with two equivalent Ni(1)O6 octahedra. In the second Co site, Co(2) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(5) atoms to form CoO6 octahedra that share edges with two equivalent Ni(1)O6 octahedra and edges with four equivalent Co(1)O6 octahedra. Ni(1) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NiO6 octahedra that share edges with two equivalent Co(2)O6 octahedra and edges with four equivalent Co(1)O6 octahedra. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Co(1), one Co(2), and one Ni(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to two equivalent Co(1) and one Ni(1) atom. In the third O site, O(3) is bonded to one Mg(1), one Co(2), and two equivalent Co(1) atoms to form a mixture of distorted corner and edge-sharing OMgCo3 trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Co(2) and two equivalent Co(1) atoms. In the fifth O site, O(5) is bonded to one Mg(1), one Co(1), one Co(2), and one Ni(1) atom to form OMgCo2Ni trigonal pyramids that share  a cornercorner with one O(5)MgCo2Ni trigonal pyramid,  a cornercorner with one O(6)MgCo2Ni trigonal pyramid,  a cornercorner with one O(3)MgCo3 trigonal pyramid,  an edgeedge with one O(5)MgCo2Ni trigonal pyramid,  an edgeedge with one O(6)MgCo2Ni trigonal pyramid, and  an edgeedge with one O(3)MgCo3 trigonal pyramid. In the sixth O site, O(6) is bonded to one Mg(1), two equivalent Co(1), and one Ni(1) atom to form OMgCo2Ni trigonal pyramids that share  a cornercorner with one O(3)MgCo3 trigonal pyramid, corners with two equivalent O(5)MgCo2Ni trigonal pyramids,  an edgeedge with one O(3)MgCo3 trigonal pyramid, and edges with two equivalent O(5)MgCo2Ni trigonal pyramids.
MgCo3NiO8 is beta indium sulfide-derived structured and crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one O(3), one O(6), and two equivalent O(5) atoms. The Mg(1)-O(3) bond length is 1.94 Å. The Mg(1)-O(6) bond length is 2.08 Å. Both Mg(1)-O(5) bond lengths are 2.10 Å. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form CoO6 octahedra that share edges with two equivalent Co(1)O6 octahedra, edges with two equivalent Co(2)O6 octahedra, and edges with two equivalent Ni(1)O6 octahedra. The Co(1)-O(1) bond length is 1.86 Å. The Co(1)-O(2) bond length is 1.85 Å. The Co(1)-O(3) bond length is 1.95 Å. The Co(1)-O(4) bond length is 1.88 Å. The Co(1)-O(5) bond length is 1.92 Å. The Co(1)-O(6) bond length is 1.91 Å. In the second Co site, Co(2) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(5) atoms to form CoO6 octahedra that share edges with two equivalent Ni(1)O6 octahedra and edges with four equivalent Co(1)O6 octahedra. The Co(2)-O(3) bond length is 1.95 Å. The Co(2)-O(4) bond length is 1.88 Å. Both Co(2)-O(1) bond lengths are 1.85 Å. Both Co(2)-O(5) bond lengths are 1.91 Å. Ni(1) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(5) atoms to form NiO6 octahedra that share edges with two equivalent Co(2)O6 octahedra and edges with four equivalent Co(1)O6 octahedra. The Ni(1)-O(2) bond length is 1.93 Å. The Ni(1)-O(6) bond length is 2.02 Å. Both Ni(1)-O(1) bond lengths are 1.93 Å. Both Ni(1)-O(5) bond lengths are 2.00 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Co(1), one Co(2), and one Ni(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to two equivalent Co(1) and one Ni(1) atom. In the third O site, O(3) is bonded to one Mg(1), one Co(2), and two equivalent Co(1) atoms to form a mixture of distorted corner and edge-sharing OMgCo3 trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Co(2) and two equivalent Co(1) atoms. In the fifth O site, O(5) is bonded to one Mg(1), one Co(1), one Co(2), and one Ni(1) atom to form OMgCo2Ni trigonal pyramids that share  a cornercorner with one O(5)MgCo2Ni trigonal pyramid,  a cornercorner with one O(6)MgCo2Ni trigonal pyramid,  a cornercorner with one O(3)MgCo3 trigonal pyramid,  an edgeedge with one O(5)MgCo2Ni trigonal pyramid,  an edgeedge with one O(6)MgCo2Ni trigonal pyramid, and  an edgeedge with one O(3)MgCo3 trigonal pyramid. In the sixth O site, O(6) is bonded to one Mg(1), two equivalent Co(1), and one Ni(1) atom to form OMgCo2Ni trigonal pyramids that share  a cornercorner with one O(3)MgCo3 trigonal pyramid, corners with two equivalent O(5)MgCo2Ni trigonal pyramids,  an edgeedge with one O(3)MgCo3 trigonal pyramid, and edges with two equivalent O(5)MgCo2Ni trigonal pyramids.