LaMg2Ni9 crystallizes in the trigonal R-3m space group. Mg(1) is bonded in a 12-coordinate geometry to three equivalent Ni(3) and nine equivalent Ni(1) atoms. La(1) is bonded in a distorted hexagonal planar geometry to six equivalent Ni(2) and twelve equivalent Ni(1) atoms. There are three inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to three equivalent Mg(1), two equivalent La(1), one Ni(3), two equivalent Ni(2), and four equivalent Ni(1) atoms to form NiLa2Mg3Ni7 cuboctahedra that share corners with two equivalent Ni(3)Mg6Ni6 cuboctahedra, corners with fifteen equivalent Ni(1)La2Mg3Ni7 cuboctahedra, edges with eight equivalent Ni(1)La2Mg3Ni7 cuboctahedra, faces with three equivalent Ni(3)Mg6Ni6 cuboctahedra, and faces with eleven equivalent Ni(1)La2Mg3Ni7 cuboctahedra. In the second Ni site, Ni(2) is bonded in a 12-coordinate geometry to three equivalent La(1), three equivalent Ni(2), and six equivalent Ni(1) atoms. In the third Ni site, Ni(3) is bonded to six equivalent Mg(1) and six equivalent Ni(1) atoms to form NiMg6Ni6 cuboctahedra that share corners with twelve equivalent Ni(1)La2Mg3Ni7 cuboctahedra, edges with six equivalent Ni(3)Mg6Ni6 cuboctahedra, and faces with eighteen equivalent Ni(1)La2Mg3Ni7 cuboctahedra.
LaMg2Ni9 crystallizes in the trigonal R-3m space group. Mg(1) is bonded in a 12-coordinate geometry to three equivalent Ni(3) and nine equivalent Ni(1) atoms. All Mg(1)-Ni(3) bond lengths are 2.85 Å. There are three shorter (2.79 Å) and six longer (2.81 Å) Mg(1)-Ni(1) bond lengths. La(1) is bonded in a distorted hexagonal planar geometry to six equivalent Ni(2) and twelve equivalent Ni(1) atoms. All La(1)-Ni(2) bond lengths are 2.81 Å. All La(1)-Ni(1) bond lengths are 3.15 Å. There are three inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to three equivalent Mg(1), two equivalent La(1), one Ni(3), two equivalent Ni(2), and four equivalent Ni(1) atoms to form NiLa2Mg3Ni7 cuboctahedra that share corners with two equivalent Ni(3)Mg6Ni6 cuboctahedra, corners with fifteen equivalent Ni(1)La2Mg3Ni7 cuboctahedra, edges with eight equivalent Ni(1)La2Mg3Ni7 cuboctahedra, faces with three equivalent Ni(3)Mg6Ni6 cuboctahedra, and faces with eleven equivalent Ni(1)La2Mg3Ni7 cuboctahedra. The Ni(1)-Ni(3) bond length is 2.38 Å. Both Ni(1)-Ni(2) bond lengths are 2.45 Å. There are two shorter (2.41 Å) and two longer (2.45 Å) Ni(1)-Ni(1) bond lengths. In the second Ni site, Ni(2) is bonded in a 12-coordinate geometry to three equivalent La(1), three equivalent Ni(2), and six equivalent Ni(1) atoms. All Ni(2)-Ni(2) bond lengths are 2.81 Å. In the third Ni site, Ni(3) is bonded to six equivalent Mg(1) and six equivalent Ni(1) atoms to form NiMg6Ni6 cuboctahedra that share corners with twelve equivalent Ni(1)La2Mg3Ni7 cuboctahedra, edges with six equivalent Ni(3)Mg6Ni6 cuboctahedra, and faces with eighteen equivalent Ni(1)La2Mg3Ni7 cuboctahedra.