MgNb4(NiO6)2 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(3) and two equivalent O(5) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 68-70°. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra, corners with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 46°. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms to form distorted NbO6 octahedra that share corners with four equivalent Nb(4)O6 octahedra, corners with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-57°. In the third Nb site, Nb(3) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(6) atoms to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Ni(1)O6 octahedra,  an edgeedge with one Nb(4)O6 octahedra, and  an edgeedge with one Ni(2)O6 octahedra. The corner-sharing octahedral tilt angles are 46°. In the fourth Nb site, Nb(4) is bonded to two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms to form distorted NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, corners with four equivalent Ni(1)O6 octahedra,  an edgeedge with one Nb(3)O6 octahedra,  an edgeedge with one Ni(2)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-57°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra, corners with four equivalent Nb(4)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Nb(1)O6 octahedra, and  an edgeedge with one Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-57°. In the second Ni site, Ni(2) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form NiO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Nb(3)O6 octahedra, and  an edgeedge with one Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-57°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(3), and one Ni(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Ni(2) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Nb(4), and one Ni(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(3), and one Ni(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Nb(4), and one Ni(2) atom. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Nb(3), one Nb(4), and one Ni(1) atom.
MgNb4(NiO6)2 crystallizes in the monoclinic P2 space group. Mg(1) is bonded to two equivalent O(3) and two equivalent O(5) atoms to form distorted MgO4 trigonal pyramids that share corners with two equivalent Ni(1)O6 octahedra, corners with two equivalent Ni(2)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 68-70°. Both Mg(1)-O(3) bond lengths are 1.90 Å. Both Mg(1)-O(5) bond lengths are 1.90 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra, corners with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 46°. Both Nb(1)-O(1) bond lengths are 2.02 Å. Both Nb(1)-O(2) bond lengths are 2.13 Å. Both Nb(1)-O(4) bond lengths are 2.05 Å. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(5) atoms to form distorted NbO6 octahedra that share corners with four equivalent Nb(4)O6 octahedra, corners with four equivalent Ni(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Ni(1)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-57°. Both Nb(2)-O(2) bond lengths are 1.95 Å. Both Nb(2)-O(3) bond lengths are 2.28 Å. Both Nb(2)-O(5) bond lengths are 2.04 Å. In the third Nb site, Nb(3) is bonded to two equivalent O(1), two equivalent O(4), and two equivalent O(6) atoms to form NbO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Ni(1)O6 octahedra,  an edgeedge with one Nb(4)O6 octahedra, and  an edgeedge with one Ni(2)O6 octahedra. The corner-sharing octahedral tilt angles are 46°. Both Nb(3)-O(1) bond lengths are 2.05 Å. Both Nb(3)-O(4) bond lengths are 2.02 Å. Both Nb(3)-O(6) bond lengths are 2.13 Å. In the fourth Nb site, Nb(4) is bonded to two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms to form distorted NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, corners with four equivalent Ni(1)O6 octahedra,  an edgeedge with one Nb(3)O6 octahedra,  an edgeedge with one Ni(2)O6 octahedra, and edges with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 39-57°. Both Nb(4)-O(3) bond lengths are 2.04 Å. Both Nb(4)-O(5) bond lengths are 2.28 Å. Both Nb(4)-O(6) bond lengths are 1.94 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with four equivalent Nb(3)O6 octahedra, corners with four equivalent Nb(4)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Nb(1)O6 octahedra, and  an edgeedge with one Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-57°. Both Ni(1)-O(1) bond lengths are 2.14 Å. Both Ni(1)-O(3) bond lengths are 2.18 Å. Both Ni(1)-O(6) bond lengths are 2.05 Å. In the second Ni site, Ni(2) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form NiO6 octahedra that share corners with four equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, corners with two equivalent Mg(1)O4 trigonal pyramids,  an edgeedge with one Nb(3)O6 octahedra, and  an edgeedge with one Nb(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-57°. Both Ni(2)-O(2) bond lengths are 2.06 Å. Both Ni(2)-O(4) bond lengths are 2.14 Å. Both Ni(2)-O(5) bond lengths are 2.18 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(3), and one Ni(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Ni(2) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Nb(4), and one Ni(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(3), and one Ni(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Nb(4), and one Ni(2) atom. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Nb(3), one Nb(4), and one Ni(1) atom.