Mg5(SiN3)2 crystallizes in the monoclinic C2/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent N(1) and two equivalent N(3) atoms to form distorted MgN4 trigonal pyramids that share corners with two equivalent Mg(2)N5 square pyramids, corners with four equivalent Si(1)N4 tetrahedra, and edges with two equivalent Mg(2)N5 square pyramids. In the second Mg site, Mg(2) is bonded to one N(1), two equivalent N(2), and two equivalent N(3) atoms to form distorted MgN5 square pyramids that share corners with three equivalent Si(1)N4 tetrahedra,  a cornercorner with one Mg(1)N4 trigonal pyramid, edges with two equivalent Mg(2)N5 square pyramids, edges with two equivalent Si(1)N4 tetrahedra, and  an edgeedge with one Mg(1)N4 trigonal pyramid. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one N(2), two equivalent N(1), and two equivalent N(3) atoms. Si(1) is bonded to one N(1), one N(3), and two equivalent N(2) atoms to form SiN4 tetrahedra that share corners with three equivalent Mg(2)N5 square pyramids, corners with two equivalent Mg(1)N4 trigonal pyramids, edges with two equivalent Mg(2)N5 square pyramids, and  an edgeedge with one Si(1)N4 tetrahedra. There are three inequivalent N sites. In the first N site, N(2) is bonded in a 5-coordinate geometry to one Mg(3), two equivalent Mg(2), and two equivalent Si(1) atoms. In the second N site, N(3) is bonded to one Mg(1), two equivalent Mg(2), two equivalent Mg(3), and one Si(1) atom to form distorted NMg5Si octahedra that share corners with five equivalent N(1)Mg4Si trigonal bipyramids,  an edgeedge with one N(3)Mg5Si octahedra, edges with two equivalent N(1)Mg4Si trigonal bipyramids, and  a faceface with one N(3)Mg5Si octahedra. In the third N site, N(1) is bonded to one Mg(1), one Mg(2), two equivalent Mg(3), and one Si(1) atom to form distorted NMg4Si trigonal bipyramids that share corners with five equivalent N(3)Mg5Si octahedra, corners with three equivalent N(1)Mg4Si trigonal bipyramids, and edges with two equivalent N(3)Mg5Si octahedra. The corner-sharing octahedral tilt angles range from 4-72°.
Mg5(SiN3)2 crystallizes in the monoclinic C2/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent N(1) and two equivalent N(3) atoms to form distorted MgN4 trigonal pyramids that share corners with two equivalent Mg(2)N5 square pyramids, corners with four equivalent Si(1)N4 tetrahedra, and edges with two equivalent Mg(2)N5 square pyramids. Both Mg(1)-N(1) bond lengths are 2.04 Å. Both Mg(1)-N(3) bond lengths are 2.27 Å. In the second Mg site, Mg(2) is bonded to one N(1), two equivalent N(2), and two equivalent N(3) atoms to form distorted MgN5 square pyramids that share corners with three equivalent Si(1)N4 tetrahedra,  a cornercorner with one Mg(1)N4 trigonal pyramid, edges with two equivalent Mg(2)N5 square pyramids, edges with two equivalent Si(1)N4 tetrahedra, and  an edgeedge with one Mg(1)N4 trigonal pyramid. The Mg(2)-N(1) bond length is 2.50 Å. There is one shorter (2.17 Å) and one longer (2.20 Å) Mg(2)-N(2) bond length. There is one shorter (2.13 Å) and one longer (2.49 Å) Mg(2)-N(3) bond length. In the third Mg site, Mg(3) is bonded in a 5-coordinate geometry to one N(2), two equivalent N(1), and two equivalent N(3) atoms. The Mg(3)-N(2) bond length is 2.44 Å. There is one shorter (2.10 Å) and one longer (2.17 Å) Mg(3)-N(1) bond length. There is one shorter (2.13 Å) and one longer (2.28 Å) Mg(3)-N(3) bond length. Si(1) is bonded to one N(1), one N(3), and two equivalent N(2) atoms to form SiN4 tetrahedra that share corners with three equivalent Mg(2)N5 square pyramids, corners with two equivalent Mg(1)N4 trigonal pyramids, edges with two equivalent Mg(2)N5 square pyramids, and  an edgeedge with one Si(1)N4 tetrahedra. The Si(1)-N(1) bond length is 1.72 Å. The Si(1)-N(3) bond length is 1.72 Å. Both Si(1)-N(2) bond lengths are 1.80 Å. There are three inequivalent N sites. In the first N site, N(2) is bonded in a 5-coordinate geometry to one Mg(3), two equivalent Mg(2), and two equivalent Si(1) atoms. In the second N site, N(3) is bonded to one Mg(1), two equivalent Mg(2), two equivalent Mg(3), and one Si(1) atom to form distorted NMg5Si octahedra that share corners with five equivalent N(1)Mg4Si trigonal bipyramids,  an edgeedge with one N(3)Mg5Si octahedra, edges with two equivalent N(1)Mg4Si trigonal bipyramids, and  a faceface with one N(3)Mg5Si octahedra. In the third N site, N(1) is bonded to one Mg(1), one Mg(2), two equivalent Mg(3), and one Si(1) atom to form distorted NMg4Si trigonal bipyramids that share corners with five equivalent N(3)Mg5Si octahedra, corners with three equivalent N(1)Mg4Si trigonal bipyramids, and edges with two equivalent N(3)Mg5Si octahedra. The corner-sharing octahedral tilt angles range from 4-72°.