Mg(NiO2)4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(4), two equivalent O(6), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Ni(4)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Mg(1)O5 square pyramids, and  a faceface with one Ni(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-55°. 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 NiO6 octahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles range from 50-55°. 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, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(3)O6 octahedra, and  a faceface with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 50-56°. In the third Ni site, Ni(3) is bonded to one O(1), one O(6), two equivalent O(2), and two equivalent O(8) atoms to form NiO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-55°. 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 Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 51-56°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Ni(3) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded in a trigonal planar geometry to one Ni(1) and two equivalent Ni(3) atoms. In the third O site, O(3) is bonded in a trigonal planar geometry to one Ni(4) and two equivalent Ni(2) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form OMgNi3 trigonal pyramids that share corners with two equivalent O(4)MgNi3 trigonal pyramids, edges with two equivalent O(7)Mg2Ni3 square pyramids, and edges with two equivalent O(6)Mg2Ni3 trigonal bipyramids. 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 to two equivalent Mg(1), one Ni(3), and two equivalent Ni(2) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with two equivalent O(7)Mg2Ni3 square pyramids,  an edgeedge with one O(7)Mg2Ni3 square pyramid, edges with two equivalent O(6)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(4)MgNi3 trigonal pyramids. In the seventh O site, O(7) is bonded to two equivalent Mg(1), one Ni(1), and two equivalent Ni(4) atoms to form OMg2Ni3 square pyramids that share corners with two equivalent O(6)Mg2Ni3 trigonal bipyramids, edges with two equivalent O(7)Mg2Ni3 square pyramids,  an edgeedge with one O(6)Mg2Ni3 trigonal bipyramid, and edges with two equivalent O(4)MgNi3 trigonal pyramids. 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(4), two equivalent O(6), and two equivalent O(7) atoms to form distorted MgO5 square pyramids that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Ni(4)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Mg(1)O5 square pyramids, and  a faceface with one Ni(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 10-55°. The Mg(1)-O(4) bond length is 2.16 Å. Both Mg(1)-O(6) bond lengths are 2.05 Å. Both Mg(1)-O(7) bond lengths are 2.04 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 NiO6 octahedra that share corners with two equivalent Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles range from 50-55°. The Ni(1)-O(2) bond length is 1.88 Å. The Ni(1)-O(7) bond length is 1.97 Å. Both Ni(1)-O(4) bond lengths are 1.89 Å. Both Ni(1)-O(5) bond lengths are 1.92 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, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(2)O6 octahedra, edges with two equivalent Ni(3)O6 octahedra, and  a faceface with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 50-56°. The Ni(2)-O(4) bond length is 1.94 Å. The Ni(2)-O(8) bond length is 1.94 Å. Both Ni(2)-O(3) bond lengths are 1.90 Å. Both Ni(2)-O(6) bond lengths are 2.09 Å. In the third Ni site, Ni(3) is bonded to one O(1), one O(6), two equivalent O(2), and two equivalent O(8) atoms to form NiO6 octahedra that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-55°. The Ni(3)-O(1) bond length is 1.85 Å. The Ni(3)-O(6) bond length is 1.93 Å. Both Ni(3)-O(2) bond lengths are 1.90 Å. Both Ni(3)-O(8) bond lengths are 1.91 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 Ni(2)O6 octahedra, corners with two equivalent Ni(3)O6 octahedra, corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra, and  an edgeedge with one Mg(1)O5 square pyramid. The corner-sharing octahedral tilt angles range from 51-56°. The Ni(4)-O(3) bond length is 1.87 Å. The Ni(4)-O(5) bond length is 1.94 Å. Both Ni(4)-O(1) bond lengths are 1.90 Å. Both Ni(4)-O(7) bond lengths are 2.05 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Ni(3) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded in a trigonal planar geometry to one Ni(1) and two equivalent Ni(3) atoms. In the third O site, O(3) is bonded in a trigonal planar geometry to one Ni(4) and two equivalent Ni(2) atoms. In the fourth O site, O(4) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form OMgNi3 trigonal pyramids that share corners with two equivalent O(4)MgNi3 trigonal pyramids, edges with two equivalent O(7)Mg2Ni3 square pyramids, and edges with two equivalent O(6)Mg2Ni3 trigonal bipyramids. 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 to two equivalent Mg(1), one Ni(3), and two equivalent Ni(2) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with two equivalent O(7)Mg2Ni3 square pyramids,  an edgeedge with one O(7)Mg2Ni3 square pyramid, edges with two equivalent O(6)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(4)MgNi3 trigonal pyramids. In the seventh O site, O(7) is bonded to two equivalent Mg(1), one Ni(1), and two equivalent Ni(4) atoms to form OMg2Ni3 square pyramids that share corners with two equivalent O(6)Mg2Ni3 trigonal bipyramids, edges with two equivalent O(7)Mg2Ni3 square pyramids,  an edgeedge with one O(6)Mg2Ni3 trigonal bipyramid, and edges with two equivalent O(4)MgNi3 trigonal pyramids. 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.