SrMg(NiO2)4 crystallizes in the monoclinic Cm space group. Sr(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form SrO6 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 1-19°. Mg(1) is bonded in a distorted rectangular see-saw-like 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 Sr(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 1-19°. 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 Sr(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 Sr(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 Sr(1), one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form OSrMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with four equivalent O(6)SrMgNi3 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 Sr(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 Sr(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 Sr(1), one Mg(1), one Ni(1), one Ni(2), and one Ni(3) atom to form OSrMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with two equivalent O(1)SrMgNi3 trigonal bipyramids, edges with two equivalent O(6)SrMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid.
SrMg(NiO2)4 crystallizes in the monoclinic Cm space group. Sr(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(6) atoms to form SrO6 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 1-19°. The Sr(1)-O(1) bond length is 2.41 Å. The Sr(1)-O(3) bond length is 2.39 Å. Both Sr(1)-O(2) bond lengths are 2.44 Å. Both Sr(1)-O(6) bond lengths are 2.44 Å. Mg(1) is bonded in a distorted rectangular see-saw-like 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 2.03 Å. Both Mg(1)-O(6) bond lengths are 2.06 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 Sr(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 1-19°. The Ni(1)-O(1) bond length is 1.90 Å. The Ni(1)-O(3) bond length is 1.84 Å. Both Ni(1)-O(2) bond lengths are 1.96 Å. Both Ni(1)-O(6) bond lengths are 2.07 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 Sr(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 1.94 Å. The Ni(2)-O(5) bond length is 2.01 Å. Both Ni(2)-O(2) bond lengths are 2.04 Å. Both Ni(2)-O(6) bond lengths are 2.11 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 Sr(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 2.19 Å. The Ni(3)-O(2) bond length is 1.89 Å. The Ni(3)-O(3) bond length is 2.10 Å. The Ni(3)-O(4) bond length is 1.92 Å. The Ni(3)-O(5) bond length is 2.00 Å. The Ni(3)-O(6) bond length is 1.95 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Sr(1), one Mg(1), one Ni(1), and two equivalent Ni(3) atoms to form OSrMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with four equivalent O(6)SrMgNi3 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 Sr(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 Sr(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 Sr(1), one Mg(1), one Ni(1), one Ni(2), and one Ni(3) atom to form OSrMgNi3 trigonal bipyramids that share  a cornercorner with one O(5)MgNi3 trigonal pyramid, edges with two equivalent O(1)SrMgNi3 trigonal bipyramids, edges with two equivalent O(6)SrMgNi3 trigonal bipyramids, and  an edgeedge with one O(5)MgNi3 trigonal pyramid.