Mg(NiO2)4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra,  a faceface with one Ni(1)O6 octahedra, and  a faceface with one Ni(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-41°. There are four inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), one O(7), two equivalent O(4), and two equivalent O(5) atoms to form distorted NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-55°. In the second Ni site, Ni(2) is bonded to one O(4), one O(8), two equivalent O(3), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-55°. In the third Ni site, Ni(3) is bonded to one O(6), two equivalent O(2), and two equivalent O(8) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 50-72°. In the fourth Ni site, Ni(4) is bonded to one O(3), one O(5), two equivalent O(1), and two equivalent O(7) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-49°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded to one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form distorted OMgNi3 tetrahedra that share corners with two equivalent O(2)MgNi3 tetrahedra,  a cornercorner with one O(3)MgNi3 trigonal pyramid, and edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Ni(4), and two equivalent Ni(2) atoms to form distorted OMgNi3 trigonal pyramids that share  a cornercorner with one O(2)MgNi3 tetrahedra, corners with two equivalent O(3)MgNi3 trigonal pyramids, and edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form distorted OMg2Ni3 trigonal bipyramids that share edges with two equivalent O(2)MgNi3 tetrahedra, edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)MgNi3 trigonal pyramids. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Ni(4) and two equivalent Ni(1) atoms. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Ni(3) and two equivalent Ni(2) atoms. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Ni(1) and two equivalent Ni(4) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Ni(2) and two equivalent Ni(3) atoms.
Mg(NiO2)4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra,  a faceface with one Ni(1)O6 octahedra, and  a faceface with one Ni(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-41°. The Mg(1)-O(2) bond length is 1.98 Å. The Mg(1)-O(3) bond length is 2.03 Å. Both Mg(1)-O(1) bond lengths are 2.05 Å. Both Mg(1)-O(4) bond lengths are 2.14 Å. There are four inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), one O(7), two equivalent O(4), and two equivalent O(5) atoms to form distorted NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-55°. The Ni(1)-O(2) bond length is 2.24 Å. The Ni(1)-O(7) bond length is 2.10 Å. Both Ni(1)-O(4) bond lengths are 1.96 Å. Both Ni(1)-O(5) bond lengths are 1.88 Å. In the second Ni site, Ni(2) is bonded to one O(4), one O(8), two equivalent O(3), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-55°. The Ni(2)-O(4) bond length is 2.02 Å. The Ni(2)-O(8) bond length is 1.83 Å. Both Ni(2)-O(3) bond lengths are 1.88 Å. Both Ni(2)-O(6) bond lengths are 1.87 Å. In the third Ni site, Ni(3) is bonded to one O(6), two equivalent O(2), and two equivalent O(8) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 50-72°. The Ni(3)-O(6) bond length is 2.06 Å. Both Ni(3)-O(2) bond lengths are 1.88 Å. Both Ni(3)-O(8) bond lengths are 1.89 Å. In the fourth Ni site, Ni(4) is bonded to one O(3), one O(5), two equivalent O(1), and two equivalent O(7) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-49°. The Ni(4)-O(3) bond length is 1.97 Å. The Ni(4)-O(5) bond length is 1.83 Å. Both Ni(4)-O(1) bond lengths are 1.93 Å. Both Ni(4)-O(7) bond lengths are 1.86 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded to one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form distorted OMgNi3 tetrahedra that share corners with two equivalent O(2)MgNi3 tetrahedra,  a cornercorner with one O(3)MgNi3 trigonal pyramid, and edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(1), one Ni(4), and two equivalent Ni(2) atoms to form distorted OMgNi3 trigonal pyramids that share  a cornercorner with one O(2)MgNi3 tetrahedra, corners with two equivalent O(3)MgNi3 trigonal pyramids, and edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids. In the fourth O site, O(4) is bonded to two equivalent Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form distorted OMg2Ni3 trigonal bipyramids that share edges with two equivalent O(2)MgNi3 tetrahedra, edges with two equivalent O(4)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)MgNi3 trigonal pyramids. In the fifth O site, O(5) is bonded in a distorted trigonal non-coplanar geometry to one Ni(4) and two equivalent Ni(1) atoms. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Ni(3) and two equivalent Ni(2) atoms. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Ni(1) and two equivalent Ni(4) atoms. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Ni(2) and two equivalent Ni(3) atoms.