MgNiO2 crystallizes in the triclinic P-1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO5 trigonal bipyramids that share corners with two equivalent Ni(2)O5 trigonal bipyramids, corners with four equivalent Ni(1)O5 trigonal bipyramids,  an edgeedge with one Ni(1)O5 trigonal bipyramid, edges with two equivalent Mg(1)O5 trigonal bipyramids, and  a faceface with one Ni(2)O5 trigonal bipyramid. In the second Mg site, Mg(2) is bonded in a distorted rectangular see-saw-like geometry to one O(3), one O(4), and two equivalent O(2) atoms. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(3) atoms to form distorted NiO5 trigonal bipyramids that share corners with four equivalent Mg(1)O5 trigonal bipyramids, corners with four equivalent Ni(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Ni(1)O5 trigonal bipyramids. In the second Ni site, Ni(2) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with four equivalent Ni(1)O5 trigonal bipyramids, edges with two equivalent Ni(2)O5 trigonal bipyramids, and  a faceface with one Mg(1)O5 trigonal bipyramid. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form OMgNi3 tetrahedra that share corners with two equivalent O(1)MgNi3 tetrahedra, corners with four equivalent O(2)Mg2Ni3 trigonal bipyramids, corners with four equivalent O(3)Mg3Ni2 trigonal bipyramids, and  an edgeedge with one O(3)Mg3Ni2 trigonal bipyramid. In the second O site, O(2) is bonded to two equivalent Mg(2), one Ni(1), and two equivalent Ni(2) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with four equivalent O(1)MgNi3 tetrahedra, edges with two equivalent O(2)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)Mg3Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and two equivalent Ni(1) atoms to form distorted OMg3Ni2 trigonal bipyramids that share corners with four equivalent O(1)MgNi3 tetrahedra,  an edgeedge with one O(1)MgNi3 tetrahedra, edges with two equivalent O(2)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)Mg3Ni2 trigonal bipyramids. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Mg(2), two equivalent Mg(1), and two equivalent Ni(2) atoms.
MgNiO2 crystallizes in the triclinic P-1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO5 trigonal bipyramids that share corners with two equivalent Ni(2)O5 trigonal bipyramids, corners with four equivalent Ni(1)O5 trigonal bipyramids,  an edgeedge with one Ni(1)O5 trigonal bipyramid, edges with two equivalent Mg(1)O5 trigonal bipyramids, and  a faceface with one Ni(2)O5 trigonal bipyramid. The Mg(1)-O(1) bond length is 2.02 Å. Both Mg(1)-O(3) bond lengths are 2.04 Å. There is one shorter (2.13 Å) and one longer (2.14 Å) Mg(1)-O(4) bond length. In the second Mg site, Mg(2) is bonded in a distorted rectangular see-saw-like geometry to one O(3), one O(4), and two equivalent O(2) atoms. The Mg(2)-O(3) bond length is 1.97 Å. The Mg(2)-O(4) bond length is 1.93 Å. There is one shorter (1.98 Å) and one longer (2.02 Å) Mg(2)-O(2) bond length. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(3) atoms to form distorted NiO5 trigonal bipyramids that share corners with four equivalent Mg(1)O5 trigonal bipyramids, corners with four equivalent Ni(2)O5 trigonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Ni(1)O5 trigonal bipyramids. The Ni(1)-O(2) bond length is 2.12 Å. There is one shorter (1.93 Å) and one longer (1.97 Å) Ni(1)-O(1) bond length. There is one shorter (2.19 Å) and one longer (2.22 Å) Ni(1)-O(3) bond length. In the second Ni site, Ni(2) is bonded to one O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted NiO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with four equivalent Ni(1)O5 trigonal bipyramids, edges with two equivalent Ni(2)O5 trigonal bipyramids, and  a faceface with one Mg(1)O5 trigonal bipyramid. The Ni(2)-O(1) bond length is 1.98 Å. There is one shorter (2.08 Å) and one longer (2.12 Å) Ni(2)-O(2) bond length. Both Ni(2)-O(4) bond lengths are 2.15 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(1) atoms to form OMgNi3 tetrahedra that share corners with two equivalent O(1)MgNi3 tetrahedra, corners with four equivalent O(2)Mg2Ni3 trigonal bipyramids, corners with four equivalent O(3)Mg3Ni2 trigonal bipyramids, and  an edgeedge with one O(3)Mg3Ni2 trigonal bipyramid. In the second O site, O(2) is bonded to two equivalent Mg(2), one Ni(1), and two equivalent Ni(2) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with four equivalent O(1)MgNi3 tetrahedra, edges with two equivalent O(2)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)Mg3Ni2 trigonal bipyramids. In the third O site, O(3) is bonded to one Mg(2), two equivalent Mg(1), and two equivalent Ni(1) atoms to form distorted OMg3Ni2 trigonal bipyramids that share corners with four equivalent O(1)MgNi3 tetrahedra,  an edgeedge with one O(1)MgNi3 tetrahedra, edges with two equivalent O(2)Mg2Ni3 trigonal bipyramids, and edges with two equivalent O(3)Mg3Ni2 trigonal bipyramids. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Mg(2), two equivalent Mg(1), and two equivalent Ni(2) atoms.