MgMn4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-6°. There are two inequivalent Mn sites. In the first Mn site, Mn(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 Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(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 edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. 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 corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-6°. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(1), one Mn(2), one Ni(1), and one Ni(2) atom to form OMgMn2Ni2 trigonal bipyramids that share  a cornercorner with one O(3)MgMn3Ni square pyramid, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with five equivalent O(1)MgMn2Ni2 trigonal bipyramids, edges with three equivalent O(3)MgMn3Ni square pyramids,  an edgeedge with one O(2)Mn3Ni tetrahedra, and edges with three equivalent O(1)MgMn2Ni2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mn(1), two equivalent Mn(2), and one Ni(1) atom to form OMn3Ni tetrahedra that share corners with three equivalent O(3)MgMn3Ni square pyramids, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with six equivalent O(1)MgMn2Ni2 trigonal bipyramids,  an edgeedge with one O(3)MgMn3Ni square pyramid, and edges with two equivalent O(1)MgMn2Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mn(1), two equivalent Mn(2), and one Ni(2) atom to form OMgMn3Ni square pyramids that share corners with four equivalent O(3)MgMn3Ni square pyramids, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with two equivalent O(1)MgMn2Ni2 trigonal bipyramids,  an edgeedge with one O(2)Mn3Ni tetrahedra, and edges with six equivalent O(1)MgMn2Ni2 trigonal bipyramids.
MgMn4(NiO4)2 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form MgO6 octahedra that share corners with six equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-6°. Both Mg(1)-O(3) bond lengths are 2.12 Å. All Mg(1)-O(1) bond lengths are 2.18 Å. There are two inequivalent Mn sites. In the first Mn site, Mn(1) is bonded in a rectangular see-saw-like geometry to one O(2), one O(3), and two equivalent O(1) atoms. The Mn(1)-O(2) bond length is 1.96 Å. The Mn(1)-O(3) bond length is 2.09 Å. Both Mn(1)-O(1) bond lengths are 2.15 Å. In the second Mn site, Mn(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MnO6 octahedra that share edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Mn(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(2)O6 octahedra. Both Mn(2)-O(1) bond lengths are 2.00 Å. Both Mn(2)-O(2) bond lengths are 1.93 Å. Both Mn(2)-O(3) bond lengths are 2.28 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 edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. Both Ni(1)-O(2) bond lengths are 1.99 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 corners with six equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Mn(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 4-6°. Both Ni(2)-O(3) bond lengths are 2.12 Å. All Ni(2)-O(1) bond lengths are 2.29 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Mn(1), one Mn(2), one Ni(1), and one Ni(2) atom to form OMgMn2Ni2 trigonal bipyramids that share  a cornercorner with one O(3)MgMn3Ni square pyramid, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with five equivalent O(1)MgMn2Ni2 trigonal bipyramids, edges with three equivalent O(3)MgMn3Ni square pyramids,  an edgeedge with one O(2)Mn3Ni tetrahedra, and edges with three equivalent O(1)MgMn2Ni2 trigonal bipyramids. In the second O site, O(2) is bonded to one Mn(1), two equivalent Mn(2), and one Ni(1) atom to form OMn3Ni tetrahedra that share corners with three equivalent O(3)MgMn3Ni square pyramids, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with six equivalent O(1)MgMn2Ni2 trigonal bipyramids,  an edgeedge with one O(3)MgMn3Ni square pyramid, and edges with two equivalent O(1)MgMn2Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Mn(1), two equivalent Mn(2), and one Ni(2) atom to form OMgMn3Ni square pyramids that share corners with four equivalent O(3)MgMn3Ni square pyramids, corners with three equivalent O(2)Mn3Ni tetrahedra, corners with two equivalent O(1)MgMn2Ni2 trigonal bipyramids,  an edgeedge with one O(2)Mn3Ni tetrahedra, and edges with six equivalent O(1)MgMn2Ni2 trigonal bipyramids.