Mg3Bi3N5 crystallizes in the triclinic P-1 space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one N(1), one N(5), and two equivalent N(3) atoms to form a mixture of corner and edge-sharing MgN4 tetrahedra. In the second Mg site, Mg(2) is bonded to one N(1), one N(2), and two equivalent N(4) atoms to form a mixture of distorted corner and edge-sharing MgN4 tetrahedra. In the third Mg site, Mg(3) is bonded to one N(1), one N(2), one N(4), and one N(5) atom to form a mixture of distorted corner and edge-sharing MgN4 trigonal pyramids. There are three inequivalent Bi sites. In the first Bi site, Bi(1) is bonded in a water-like geometry to one N(2) and one N(3) atom. In the second Bi site, Bi(2) is bonded in a 3-coordinate geometry to one N(4) and two equivalent N(5) atoms. In the third Bi site, Bi(3) is bonded in a distorted T-shaped geometry to one N(1), one N(2), and one N(4) atom. There are five inequivalent N sites. In the first N site, N(1) is bonded in a distorted see-saw-like geometry to one Mg(1), one Mg(2), one Mg(3), and one Bi(3) atom. In the second N site, N(2) is bonded to one Mg(2), one Mg(3), one Bi(1), and one Bi(3) atom to form a mixture of distorted corner and edge-sharing NMg2Bi2 trigonal pyramids. In the third N site, N(3) is bonded in a distorted T-shaped geometry to two equivalent Mg(1) and one Bi(1) atom. In the fourth N site, N(4) is bonded to one Mg(3), two equivalent Mg(2), one Bi(2), and one Bi(3) atom to form distorted NMg3Bi2 trigonal bipyramids that share corners with two equivalent N(2)Mg2Bi2 trigonal pyramids,  an edgeedge with one N(4)Mg3Bi2 trigonal bipyramid, and  an edgeedge with one N(2)Mg2Bi2 trigonal pyramid. In the fifth N site, N(5) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Mg(3), and two equivalent Bi(2) atoms.
Mg3Bi3N5 crystallizes in the triclinic P-1 space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one N(1), one N(5), and two equivalent N(3) atoms to form a mixture of corner and edge-sharing MgN4 tetrahedra. The Mg(1)-N(1) bond length is 2.14 Å. The Mg(1)-N(5) bond length is 2.11 Å. There is one shorter (2.11 Å) and one longer (2.13 Å) Mg(1)-N(3) bond length. In the second Mg site, Mg(2) is bonded to one N(1), one N(2), and two equivalent N(4) atoms to form a mixture of distorted corner and edge-sharing MgN4 tetrahedra. The Mg(2)-N(1) bond length is 2.08 Å. The Mg(2)-N(2) bond length is 2.22 Å. There is one shorter (2.10 Å) and one longer (2.18 Å) Mg(2)-N(4) bond length. In the third Mg site, Mg(3) is bonded to one N(1), one N(2), one N(4), and one N(5) atom to form a mixture of distorted corner and edge-sharing MgN4 trigonal pyramids. The Mg(3)-N(1) bond length is 2.13 Å. The Mg(3)-N(2) bond length is 2.10 Å. The Mg(3)-N(4) bond length is 2.20 Å. The Mg(3)-N(5) bond length is 2.09 Å. There are three inequivalent Bi sites. In the first Bi site, Bi(1) is bonded in a water-like geometry to one N(2) and one N(3) atom. The Bi(1)-N(2) bond length is 2.20 Å. The Bi(1)-N(3) bond length is 2.05 Å. In the second Bi site, Bi(2) is bonded in a 3-coordinate geometry to one N(4) and two equivalent N(5) atoms. The Bi(2)-N(4) bond length is 2.35 Å. There is one shorter (2.23 Å) and one longer (2.25 Å) Bi(2)-N(5) bond length. In the third Bi site, Bi(3) is bonded in a distorted T-shaped geometry to one N(1), one N(2), and one N(4) atom. The Bi(3)-N(1) bond length is 2.18 Å. The Bi(3)-N(2) bond length is 2.29 Å. The Bi(3)-N(4) bond length is 2.30 Å. There are five inequivalent N sites. In the first N site, N(1) is bonded in a distorted see-saw-like geometry to one Mg(1), one Mg(2), one Mg(3), and one Bi(3) atom. In the second N site, N(2) is bonded to one Mg(2), one Mg(3), one Bi(1), and one Bi(3) atom to form a mixture of distorted corner and edge-sharing NMg2Bi2 trigonal pyramids. In the third N site, N(3) is bonded in a distorted T-shaped geometry to two equivalent Mg(1) and one Bi(1) atom. In the fourth N site, N(4) is bonded to one Mg(3), two equivalent Mg(2), one Bi(2), and one Bi(3) atom to form distorted NMg3Bi2 trigonal bipyramids that share corners with two equivalent N(2)Mg2Bi2 trigonal pyramids,  an edgeedge with one N(4)Mg3Bi2 trigonal bipyramid, and  an edgeedge with one N(2)Mg2Bi2 trigonal pyramid. In the fifth N site, N(5) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Mg(3), and two equivalent Bi(2) atoms.