MgCo5SbO8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one O(4), one O(5), and two equivalent O(1) atoms. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form CoO4 tetrahedra that share corners with three equivalent Sb(1)O6 octahedra, corners with six equivalent Co(2)O6 octahedra, and  a cornercorner with one Co(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-72°. In the second Co site, Co(2) 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 CoO6 octahedra that share  a cornercorner with one Co(4)O4 tetrahedra, corners with three equivalent Co(1)O4 tetrahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Sb(1)O6 octahedra, and  an edgeedge with one Co(4)O4 tetrahedra. In the third Co site, Co(3) is bonded in a 5-coordinate geometry to one O(3), two equivalent O(1), and two equivalent O(6) atoms. In the fourth Co site, Co(4) is bonded to one O(3), one O(4), and two equivalent O(6) atoms to form distorted CoO4 tetrahedra that share corners with two equivalent Co(2)O6 octahedra, corners with two equivalent Sb(1)O6 octahedra,  a cornercorner with one Co(1)O4 tetrahedra, and edges with two equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-74°. Sb(1) is bonded to one O(2), one O(5), two equivalent O(1), and two equivalent O(6) atoms to form SbO6 octahedra that share corners with two equivalent Co(4)O4 tetrahedra, corners with three equivalent Co(1)O4 tetrahedra, and edges with four equivalent Co(2)O6 octahedra. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Mg(1), one Co(1), one Co(2), one Co(3), and one Sb(1) atom. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Co(1), two equivalent Co(2), and one Sb(1) atom. In the third O site, O(3) is bonded to one Co(1), one Co(3), one Co(4), and two equivalent Co(2) atoms to form OCo5 trigonal bipyramids that share corners with three equivalent O(4)MgCo3 tetrahedra and corners with two equivalent O(5)MgCo2Sb trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Co(4), and two equivalent Co(2) atoms to form distorted OMgCo3 tetrahedra that share corners with three equivalent O(3)Co5 trigonal bipyramids,  a cornercorner with one O(5)MgCo2Sb trigonal pyramid, and  an edgeedge with one O(5)MgCo2Sb trigonal pyramid. In the fifth O site, O(5) is bonded to one Mg(1), two equivalent Co(2), and one Sb(1) atom to form OMgCo2Sb trigonal pyramids that share  a cornercorner with one O(4)MgCo3 tetrahedra, corners with two equivalent O(3)Co5 trigonal bipyramids, and  an edgeedge with one O(4)MgCo3 tetrahedra. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Co(2), one Co(3), one Co(4), and one Sb(1) atom.
MgCo5SbO8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded in a distorted rectangular see-saw-like geometry to one O(4), one O(5), and two equivalent O(1) atoms. The Mg(1)-O(4) bond length is 1.93 Å. The Mg(1)-O(5) bond length is 1.97 Å. Both Mg(1)-O(1) bond lengths are 2.11 Å. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form CoO4 tetrahedra that share corners with three equivalent Sb(1)O6 octahedra, corners with six equivalent Co(2)O6 octahedra, and  a cornercorner with one Co(4)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-72°. The Co(1)-O(2) bond length is 2.02 Å. The Co(1)-O(3) bond length is 2.14 Å. Both Co(1)-O(1) bond lengths are 2.01 Å. In the second Co site, Co(2) 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 CoO6 octahedra that share  a cornercorner with one Co(4)O4 tetrahedra, corners with three equivalent Co(1)O4 tetrahedra, edges with two equivalent Co(2)O6 octahedra, edges with two equivalent Sb(1)O6 octahedra, and  an edgeedge with one Co(4)O4 tetrahedra. The Co(2)-O(1) bond length is 2.37 Å. The Co(2)-O(2) bond length is 2.18 Å. The Co(2)-O(3) bond length is 2.22 Å. The Co(2)-O(4) bond length is 2.06 Å. The Co(2)-O(5) bond length is 2.08 Å. The Co(2)-O(6) bond length is 2.25 Å. In the third Co site, Co(3) is bonded in a 5-coordinate geometry to one O(3), two equivalent O(1), and two equivalent O(6) atoms. The Co(3)-O(3) bond length is 2.01 Å. Both Co(3)-O(1) bond lengths are 2.15 Å. Both Co(3)-O(6) bond lengths are 2.08 Å. In the fourth Co site, Co(4) is bonded to one O(3), one O(4), and two equivalent O(6) atoms to form distorted CoO4 tetrahedra that share corners with two equivalent Co(2)O6 octahedra, corners with two equivalent Sb(1)O6 octahedra,  a cornercorner with one Co(1)O4 tetrahedra, and edges with two equivalent Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-74°. The Co(4)-O(3) bond length is 2.03 Å. The Co(4)-O(4) bond length is 1.98 Å. Both Co(4)-O(6) bond lengths are 1.99 Å. Sb(1) is bonded to one O(2), one O(5), two equivalent O(1), and two equivalent O(6) atoms to form SbO6 octahedra that share corners with two equivalent Co(4)O4 tetrahedra, corners with three equivalent Co(1)O4 tetrahedra, and edges with four equivalent Co(2)O6 octahedra. The Sb(1)-O(2) bond length is 2.11 Å. The Sb(1)-O(5) bond length is 2.14 Å. Both Sb(1)-O(1) bond lengths are 2.30 Å. Both Sb(1)-O(6) bond lengths are 2.06 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to one Mg(1), one Co(1), one Co(2), one Co(3), and one Sb(1) atom. In the second O site, O(2) is bonded in a rectangular see-saw-like geometry to one Co(1), two equivalent Co(2), and one Sb(1) atom. In the third O site, O(3) is bonded to one Co(1), one Co(3), one Co(4), and two equivalent Co(2) atoms to form OCo5 trigonal bipyramids that share corners with three equivalent O(4)MgCo3 tetrahedra and corners with two equivalent O(5)MgCo2Sb trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Co(4), and two equivalent Co(2) atoms to form distorted OMgCo3 tetrahedra that share corners with three equivalent O(3)Co5 trigonal bipyramids,  a cornercorner with one O(5)MgCo2Sb trigonal pyramid, and  an edgeedge with one O(5)MgCo2Sb trigonal pyramid. In the fifth O site, O(5) is bonded to one Mg(1), two equivalent Co(2), and one Sb(1) atom to form OMgCo2Sb trigonal pyramids that share  a cornercorner with one O(4)MgCo3 tetrahedra, corners with two equivalent O(3)Co5 trigonal bipyramids, and  an edgeedge with one O(4)MgCo3 tetrahedra. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Co(2), one Co(3), one Co(4), and one Sb(1) atom.