BaMg(NiO2)4 crystallizes in the monoclinic Cm space group. Ba(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form distorted BaO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. Mg(1) is bonded in a 4-coordinate geometry to one O(1), one O(5), and two equivalent O(6) atoms. There are three inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with six equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. In the second Ni site, Ni(2) is bonded to one O(4), one O(5), two equivalent O(2), and two equivalent O(6) atoms to form NiO6 octahedra that share edges with two equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. In the third Ni site, Ni(3) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form NiO6 octahedra that share edges with two equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O6 octahedra. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Ba(1), one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form OBaMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with four equivalent O(6)BaMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Ba(1), one Ni(1), one Ni(2), and one Ni(3) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Ba(1), one Ni(1), and two equivalent Ni(3) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ni(2) and two equivalent Ni(3) atoms. In the fifth O site, O(5) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(3) atoms to form a mixture of distorted edge and corner-sharing OMgNi3 trigonal pyramids. In the sixth O site, O(6) is bonded to one Ba(1), one Mg(1), one Ni(1), one Ni(2), and one Ni(3) atom to form OBaMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with two equivalent O(1)BaMgNi3 trigonal bipyramids, edges with two equivalent O(6)BaMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid.
BaMg(NiO2)4 crystallizes in the monoclinic Cm space group. Ba(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form distorted BaO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. The Ba(1)-O(1) bond length is 2.57 Å. The Ba(1)-O(3) bond length is 2.55 Å. Both Ba(1)-O(2) bond lengths are 2.56 Å. Both Ba(1)-O(6) bond lengths are 2.57 Å. Mg(1) is bonded in a 4-coordinate geometry to one O(1), one O(5), and two equivalent O(6) atoms. The Mg(1)-O(1) bond length is 2.07 Å. The Mg(1)-O(5) bond length is 2.04 Å. Both Mg(1)-O(6) bond lengths are 2.07 Å. There are three inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with six equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. The Ni(1)-O(1) bond length is 1.92 Å. The Ni(1)-O(3) bond length is 1.85 Å. Both Ni(1)-O(2) bond lengths are 1.85 Å. Both Ni(1)-O(6) bond lengths are 1.92 Å. In the second Ni site, Ni(2) is bonded to one O(4), one O(5), two equivalent O(2), and two equivalent O(6) atoms to form NiO6 octahedra that share edges with two equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Ni(3)O6 octahedra. The Ni(2)-O(4) bond length is 1.93 Å. The Ni(2)-O(5) bond length is 2.03 Å. Both Ni(2)-O(2) bond lengths are 2.04 Å. Both Ni(2)-O(6) bond lengths are 2.12 Å. In the third Ni site, Ni(3) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form NiO6 octahedra that share edges with two equivalent Ba(1)O6 octahedra, edges with two equivalent Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and edges with two equivalent Ni(3)O6 octahedra. The Ni(3)-O(1) bond length is 2.12 Å. The Ni(3)-O(2) bond length is 2.05 Å. The Ni(3)-O(3) bond length is 2.05 Å. The Ni(3)-O(4) bond length is 1.94 Å. The Ni(3)-O(5) bond length is 2.03 Å. The Ni(3)-O(6) bond length is 2.12 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Ba(1), one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form OBaMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with four equivalent O(6)BaMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Ba(1), one Ni(1), one Ni(2), and one Ni(3) atom. In the third O site, O(3) is bonded in a rectangular see-saw-like geometry to one Ba(1), one Ni(1), and two equivalent Ni(3) atoms. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Ni(2) and two equivalent Ni(3) atoms. In the fifth O site, O(5) is bonded to one Mg(1), one Ni(2), and two equivalent Ni(3) atoms to form a mixture of distorted edge and corner-sharing OMgNi3 trigonal pyramids. In the sixth O site, O(6) is bonded to one Ba(1), one Mg(1), one Ni(1), one Ni(2), and one Ni(3) atom to form OBaMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with two equivalent O(1)BaMgNi3 trigonal bipyramids, edges with two equivalent O(6)BaMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid.