Mg(NiO2)4 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with two equivalent Ni(2)O5 trigonal bipyramids, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Ni(2)O5 trigonal bipyramid, and  an edgeedge with one Ni(3)O5 trigonal bipyramid. 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(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(4)O6 octahedra. In the second Ni site, Ni(2) is bonded to one O(8), two equivalent O(3), and two equivalent O(6) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Ni(2)O5 trigonal bipyramids, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-61°. 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,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Ni(2)O5 trigonal bipyramids, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-61°. 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)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(4)O6 octahedra. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded to two equivalent Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form a mixture of distorted edge and corner-sharing OMg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to two equivalent Mg(1), one Ni(4), and two equivalent Ni(2) atoms to form a mixture of distorted edge and corner-sharing OMg2Ni3 trigonal bipyramids. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Ni(1) atoms. In the fifth O site, O(5) is bonded in a distorted T-shaped 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 T-shaped 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(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with two equivalent Ni(2)O5 trigonal bipyramids, corners with two equivalent Ni(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Ni(2)O5 trigonal bipyramid, and  an edgeedge with one Ni(3)O5 trigonal bipyramid. The Mg(1)-O(1) bond length is 1.98 Å. The Mg(1)-O(4) bond length is 1.98 Å. Both Mg(1)-O(2) bond lengths are 2.12 Å. Both Mg(1)-O(3) bond lengths are 2.12 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(3)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(4)O6 octahedra. The Ni(1)-O(2) bond length is 1.97 Å. The Ni(1)-O(7) bond length is 1.86 Å. Both Ni(1)-O(4) bond lengths are 1.87 Å. Both Ni(1)-O(5) bond lengths are 1.88 Å. In the second Ni site, Ni(2) is bonded to one O(8), two equivalent O(3), and two equivalent O(6) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra, corners with two equivalent Ni(4)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Ni(2)O5 trigonal bipyramids, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-61°. The Ni(2)-O(8) bond length is 2.03 Å. Both Ni(2)-O(3) bond lengths are 1.98 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,  an edgeedge with one Mg(1)O6 octahedra, edges with two equivalent Ni(2)O5 trigonal bipyramids, and edges with two equivalent Ni(3)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 33-61°. The Ni(3)-O(6) bond length is 2.02 Å. Both Ni(3)-O(2) bond lengths are 1.98 Å. Both Ni(3)-O(8) bond lengths are 1.87 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)O5 trigonal bipyramids, edges with two equivalent Mg(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with two equivalent Ni(4)O6 octahedra. The Ni(4)-O(3) bond length is 1.97 Å. The Ni(4)-O(5) bond length is 1.86 Å. Both Ni(4)-O(1) bond lengths are 1.87 Å. Both Ni(4)-O(7) bond lengths are 1.88 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Ni(4) atoms. In the second O site, O(2) is bonded to two equivalent Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form a mixture of distorted edge and corner-sharing OMg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to two equivalent Mg(1), one Ni(4), and two equivalent Ni(2) atoms to form a mixture of distorted edge and corner-sharing OMg2Ni3 trigonal bipyramids. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1) and two equivalent Ni(1) atoms. In the fifth O site, O(5) is bonded in a distorted T-shaped 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 T-shaped geometry to one Ni(2) and two equivalent Ni(3) atoms.