Mg(NiO2)2 crystallizes in the orthorhombic Cmcm space group. Mg(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. Ni(1) is bonded to one O(2), two equivalent O(3), and three equivalent O(1) atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles range from 0-55°. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1) and three equivalent Ni(1) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with four equivalent O(3)Mg2Ni4 octahedra, corners with five equivalent O(2)Mg2Ni2 tetrahedra, corners with two equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with three equivalent O(3)Mg2Ni4 octahedra,  an edgeedge with one O(2)Mg2Ni2 tetrahedra, and edges with five equivalent O(1)Mg2Ni3 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 7-44°. In the second O site, O(2) is bonded to two equivalent Mg(1) and two equivalent Ni(1) atoms to form OMg2Ni2 tetrahedra that share corners with two equivalent O(2)Mg2Ni2 tetrahedra, corners with ten equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with four equivalent O(3)Mg2Ni4 octahedra, and edges with two equivalent O(1)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to two equivalent Mg(1) and four equivalent Ni(1) atoms to form distorted OMg2Ni4 octahedra that share corners with two equivalent O(3)Mg2Ni4 octahedra, corners with eight equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with two equivalent O(3)Mg2Ni4 octahedra, edges with four equivalent O(2)Mg2Ni2 tetrahedra, and edges with six equivalent O(1)Mg2Ni3 trigonal bipyramids. The corner-sharing octahedral tilt angles are 47°.
Mg(NiO2)2 crystallizes in the orthorhombic Cmcm space group. Mg(1) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), and four equivalent O(1) atoms. Both Mg(1)-O(2) bond lengths are 2.04 Å. Both Mg(1)-O(3) bond lengths are 2.55 Å. All Mg(1)-O(1) bond lengths are 2.23 Å. Ni(1) is bonded to one O(2), two equivalent O(3), and three equivalent O(1) atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles range from 0-55°. The Ni(1)-O(2) bond length is 1.90 Å. Both Ni(1)-O(3) bond lengths are 2.04 Å. There is one shorter (1.92 Å) and two longer (2.02 Å) Ni(1)-O(1) bond lengths. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Mg(1) and three equivalent Ni(1) atoms to form distorted OMg2Ni3 trigonal bipyramids that share corners with four equivalent O(3)Mg2Ni4 octahedra, corners with five equivalent O(2)Mg2Ni2 tetrahedra, corners with two equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with three equivalent O(3)Mg2Ni4 octahedra,  an edgeedge with one O(2)Mg2Ni2 tetrahedra, and edges with five equivalent O(1)Mg2Ni3 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 7-44°. In the second O site, O(2) is bonded to two equivalent Mg(1) and two equivalent Ni(1) atoms to form OMg2Ni2 tetrahedra that share corners with two equivalent O(2)Mg2Ni2 tetrahedra, corners with ten equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with four equivalent O(3)Mg2Ni4 octahedra, and edges with two equivalent O(1)Mg2Ni3 trigonal bipyramids. In the third O site, O(3) is bonded to two equivalent Mg(1) and four equivalent Ni(1) atoms to form distorted OMg2Ni4 octahedra that share corners with two equivalent O(3)Mg2Ni4 octahedra, corners with eight equivalent O(1)Mg2Ni3 trigonal bipyramids, edges with two equivalent O(3)Mg2Ni4 octahedra, edges with four equivalent O(2)Mg2Ni2 tetrahedra, and edges with six equivalent O(1)Mg2Ni3 trigonal bipyramids. The corner-sharing octahedral tilt angles are 47°.