Mg5Sb crystallizes in the monoclinic Cm space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), three equivalent Mg(5), and two equivalent Sb(1) atoms to form distorted MgMg10Sb2 cuboctahedra that share corners with two equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with two equivalent Sb(1)Mg10 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(4)Mg11Sb cuboctahedra, corners with six equivalent Mg(2)Mg5Sb3 cuboctahedra,  an edgeedge with one Mg(2)Mg5Sb3 cuboctahedra, edges with two equivalent Mg(5)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with three equivalent Mg(3)Mg6Sb2 cuboctahedra, edges with four equivalent Sb(1)Mg10 cuboctahedra, faces with two equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(4)Mg11Sb cuboctahedra, faces with three equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Sb(1)Mg10 cuboctahedra, and faces with five equivalent Mg(5)Mg10Sb2 cuboctahedra. In the second Mg site, Mg(2) is bonded to one Mg(5), two equivalent Mg(1), two equivalent Mg(4), and three equivalent Sb(1) atoms to form distorted MgMg5Sb3 cuboctahedra that share corners with two equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with two equivalent Sb(1)Mg10 cuboctahedra, corners with four equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with seven equivalent Mg(4)Mg11Sb cuboctahedra,  an edgeedge with one Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(5)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with three equivalent Sb(1)Mg10 cuboctahedra, edges with six equivalent Mg(3)Mg6Sb2 cuboctahedra,  a faceface with one Mg(3)Mg6Sb2 cuboctahedra, faces with two equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, and faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra. In the third Mg site, Mg(3) is bonded to one Mg(1), two equivalent Mg(5), three equivalent Mg(4), and two equivalent Sb(1) atoms to form distorted MgMg6Sb2 cuboctahedra that share corners with two equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with two equivalent Mg(4)Mg11Sb cuboctahedra, corners with four equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with six equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with seven equivalent Sb(1)Mg10 cuboctahedra,  an edgeedge with one Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Sb(1)Mg10 cuboctahedra, edges with three equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with six equivalent Mg(2)Mg5Sb3 cuboctahedra,  a faceface with one Mg(2)Mg5Sb3 cuboctahedra, faces with two equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, and faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), three equivalent Mg(3), and one Sb(1) atom to form distorted MgMg11Sb cuboctahedra that share corners with two equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(4)Mg11Sb cuboctahedra, corners with seven equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with three equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with three equivalent Mg(3)Mg6Sb2 cuboctahedra, edges with four equivalent Sb(1)Mg10 cuboctahedra, edges with six equivalent Mg(5)Mg10Sb2 cuboctahedra, faces with two equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra, and faces with four equivalent Sb(1)Mg10 cuboctahedra. In the fifth Mg site, Mg(5) is bonded to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(5), three equivalent Mg(1), and two equivalent Sb(1) atoms to form distorted MgMg10Sb2 cuboctahedra that share corners with two equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with six equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with eight equivalent Sb(1)Mg10 cuboctahedra,  an edgeedge with one Mg(3)Mg6Sb2 cuboctahedra,  an edgeedge with one Sb(1)Mg10 cuboctahedra, edges with two equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with six equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(5)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(4)Mg11Sb cuboctahedra, faces with three equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Sb(1)Mg10 cuboctahedra, and faces with five equivalent Mg(1)Mg10Sb2 cuboctahedra. Sb(1) is bonded to one Mg(4), two equivalent Mg(1), two equivalent Mg(3), two equivalent Mg(5), and three equivalent Mg(2) atoms to form distorted SbMg10 cuboctahedra that share corners with two equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with two equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with four equivalent Sb(1)Mg10 cuboctahedra, corners with seven equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with eight equivalent Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Mg(3)Mg6Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with four equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with four equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Sb(1)Mg10 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra, and faces with four equivalent Mg(4)Mg11Sb cuboctahedra.
Mg5Sb crystallizes in the monoclinic Cm space group. There are five inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one Mg(3), two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), three equivalent Mg(5), and two equivalent Sb(1) atoms to form distorted MgMg10Sb2 cuboctahedra that share corners with two equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with two equivalent Sb(1)Mg10 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(4)Mg11Sb cuboctahedra, corners with six equivalent Mg(2)Mg5Sb3 cuboctahedra,  an edgeedge with one Mg(2)Mg5Sb3 cuboctahedra, edges with two equivalent Mg(5)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with three equivalent Mg(3)Mg6Sb2 cuboctahedra, edges with four equivalent Sb(1)Mg10 cuboctahedra, faces with two equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(4)Mg11Sb cuboctahedra, faces with three equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Sb(1)Mg10 cuboctahedra, and faces with five equivalent Mg(5)Mg10Sb2 cuboctahedra. The Mg(1)-Mg(3) bond length is 3.20 Å. Both Mg(1)-Mg(1) bond lengths are 3.26 Å. Both Mg(1)-Mg(2) bond lengths are 3.19 Å. Both Mg(1)-Mg(4) bond lengths are 3.18 Å. There are two shorter (3.16 Å) and one longer (3.21 Å) Mg(1)-Mg(5) bond length. Both Mg(1)-Sb(1) bond lengths are 3.28 Å. In the second Mg site, Mg(2) is bonded to one Mg(5), two equivalent Mg(1), two equivalent Mg(4), and three equivalent Sb(1) atoms to form distorted MgMg5Sb3 cuboctahedra that share corners with two equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with two equivalent Sb(1)Mg10 cuboctahedra, corners with four equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with seven equivalent Mg(4)Mg11Sb cuboctahedra,  an edgeedge with one Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(5)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with three equivalent Sb(1)Mg10 cuboctahedra, edges with six equivalent Mg(3)Mg6Sb2 cuboctahedra,  a faceface with one Mg(3)Mg6Sb2 cuboctahedra, faces with two equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, and faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra. The Mg(2)-Mg(5) bond length is 3.23 Å. Both Mg(2)-Mg(4) bond lengths are 3.19 Å. There is one shorter (3.15 Å) and two longer (3.21 Å) Mg(2)-Sb(1) bond lengths. In the third Mg site, Mg(3) is bonded to one Mg(1), two equivalent Mg(5), three equivalent Mg(4), and two equivalent Sb(1) atoms to form distorted MgMg6Sb2 cuboctahedra that share corners with two equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with two equivalent Mg(4)Mg11Sb cuboctahedra, corners with four equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with six equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with seven equivalent Sb(1)Mg10 cuboctahedra,  an edgeedge with one Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Sb(1)Mg10 cuboctahedra, edges with three equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(4)Mg11Sb cuboctahedra, edges with six equivalent Mg(2)Mg5Sb3 cuboctahedra,  a faceface with one Mg(2)Mg5Sb3 cuboctahedra, faces with two equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, and faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra. Both Mg(3)-Mg(5) bond lengths are 3.17 Å. There is one shorter (3.13 Å) and two longer (3.24 Å) Mg(3)-Mg(4) bond lengths. Both Mg(3)-Sb(1) bond lengths are 3.16 Å. In the fourth Mg site, Mg(4) is bonded to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), two equivalent Mg(5), three equivalent Mg(3), and one Sb(1) atom to form distorted MgMg11Sb cuboctahedra that share corners with two equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with six equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(4)Mg11Sb cuboctahedra, corners with seven equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with three equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with three equivalent Mg(3)Mg6Sb2 cuboctahedra, edges with four equivalent Sb(1)Mg10 cuboctahedra, edges with six equivalent Mg(5)Mg10Sb2 cuboctahedra, faces with two equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra, and faces with four equivalent Sb(1)Mg10 cuboctahedra. Both Mg(4)-Mg(4) bond lengths are 3.26 Å. Both Mg(4)-Mg(5) bond lengths are 3.21 Å. The Mg(4)-Sb(1) bond length is 3.23 Å. In the fifth Mg site, Mg(5) is bonded to one Mg(2), two equivalent Mg(3), two equivalent Mg(4), two equivalent Mg(5), three equivalent Mg(1), and two equivalent Sb(1) atoms to form distorted MgMg10Sb2 cuboctahedra that share corners with two equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with six equivalent Mg(5)Mg10Sb2 cuboctahedra, corners with six equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with eight equivalent Sb(1)Mg10 cuboctahedra,  an edgeedge with one Mg(3)Mg6Sb2 cuboctahedra,  an edgeedge with one Sb(1)Mg10 cuboctahedra, edges with two equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with six equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Mg(5)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(4)Mg11Sb cuboctahedra, faces with three equivalent Mg(2)Mg5Sb3 cuboctahedra, faces with three equivalent Mg(3)Mg6Sb2 cuboctahedra, faces with three equivalent Sb(1)Mg10 cuboctahedra, and faces with five equivalent Mg(1)Mg10Sb2 cuboctahedra. Both Mg(5)-Mg(5) bond lengths are 3.26 Å. Both Mg(5)-Sb(1) bond lengths are 3.24 Å. Sb(1) is bonded to one Mg(4), two equivalent Mg(1), two equivalent Mg(3), two equivalent Mg(5), and three equivalent Mg(2) atoms to form distorted SbMg10 cuboctahedra that share corners with two equivalent Mg(1)Mg10Sb2 cuboctahedra, corners with two equivalent Mg(2)Mg5Sb3 cuboctahedra, corners with four equivalent Sb(1)Mg10 cuboctahedra, corners with seven equivalent Mg(3)Mg6Sb2 cuboctahedra, corners with eight equivalent Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Mg(5)Mg10Sb2 cuboctahedra,  an edgeedge with one Mg(3)Mg6Sb2 cuboctahedra, edges with three equivalent Mg(2)Mg5Sb3 cuboctahedra, edges with four equivalent Mg(1)Mg10Sb2 cuboctahedra, edges with four equivalent Mg(4)Mg11Sb cuboctahedra, faces with two equivalent Sb(1)Mg10 cuboctahedra, faces with three equivalent Mg(1)Mg10Sb2 cuboctahedra, faces with three equivalent Mg(5)Mg10Sb2 cuboctahedra, and faces with four equivalent Mg(4)Mg11Sb cuboctahedra.