Mg2YbCu9 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 12-coordinate geometry to three equivalent Cu(5) and nine equivalent Cu(2) atoms. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to three equivalent Cu(4) and nine equivalent Cu(6) atoms. Yb(1) is bonded in a distorted hexagonal planar geometry to three equivalent Cu(1), three equivalent Cu(3), six equivalent Cu(2), and six equivalent Cu(6) atoms. There are six inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a 12-coordinate geometry to three equivalent Yb(1), three equivalent Cu(2), three equivalent Cu(3), and three equivalent Cu(6) atoms. In the second Cu site, Cu(2) is bonded to three equivalent Mg(1), two equivalent Yb(1), one Cu(1), one Cu(3), one Cu(5), and four equivalent Cu(2) atoms to form CuYb2Mg3Cu7 cuboctahedra that share corners with two equivalent Cu(5)Mg6Cu6 cuboctahedra, corners with six equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, corners with nine equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra,  a faceface with one Cu(6)Yb2Mg3Cu7 cuboctahedra, faces with three equivalent Cu(5)Mg6Cu6 cuboctahedra, and faces with ten equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra. In the third Cu site, Cu(3) is bonded in a 12-coordinate geometry to three equivalent Yb(1), three equivalent Cu(1), three equivalent Cu(2), and three equivalent Cu(6) atoms. In the fourth Cu site, Cu(4) is bonded to six equivalent Mg(2) and six equivalent Cu(6) atoms to form CuMg6Cu6 cuboctahedra that share corners with twelve equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, edges with six equivalent Cu(4)Mg6Cu6 cuboctahedra, and faces with eighteen equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra. In the fifth Cu site, Cu(5) is bonded to six equivalent Mg(1) and six equivalent Cu(2) atoms to form CuMg6Cu6 cuboctahedra that share corners with twelve equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with six equivalent Cu(5)Mg6Cu6 cuboctahedra, and faces with eighteen equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra. In the sixth Cu site, Cu(6) is bonded to three equivalent Mg(2), two equivalent Yb(1), one Cu(1), one Cu(3), one Cu(4), and four equivalent Cu(6) atoms to form CuYb2Mg3Cu7 cuboctahedra that share corners with two equivalent Cu(4)Mg6Cu6 cuboctahedra, corners with six equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, corners with nine equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra,  a faceface with one Cu(2)Yb2Mg3Cu7 cuboctahedra, faces with three equivalent Cu(4)Mg6Cu6 cuboctahedra, and faces with ten equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra.
Mg2YbCu9 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 12-coordinate geometry to three equivalent Cu(5) and nine equivalent Cu(2) atoms. All Mg(1)-Cu(5) bond lengths are 2.89 Å. All Mg(1)-Cu(2) bond lengths are 2.87 Å. In the second Mg site, Mg(2) is bonded in a 12-coordinate geometry to three equivalent Cu(4) and nine equivalent Cu(6) atoms. All Mg(2)-Cu(4) bond lengths are 2.89 Å. All Mg(2)-Cu(6) bond lengths are 2.86 Å. Yb(1) is bonded in a distorted hexagonal planar geometry to three equivalent Cu(1), three equivalent Cu(3), six equivalent Cu(2), and six equivalent Cu(6) atoms. All Yb(1)-Cu(1) bond lengths are 2.84 Å. All Yb(1)-Cu(3) bond lengths are 2.84 Å. All Yb(1)-Cu(2) bond lengths are 3.18 Å. All Yb(1)-Cu(6) bond lengths are 3.19 Å. There are six inequivalent Cu sites. In the first Cu site, Cu(1) is bonded in a 12-coordinate geometry to three equivalent Yb(1), three equivalent Cu(2), three equivalent Cu(3), and three equivalent Cu(6) atoms. All Cu(1)-Cu(2) bond lengths are 2.46 Å. All Cu(1)-Cu(3) bond lengths are 2.84 Å. All Cu(1)-Cu(6) bond lengths are 2.46 Å. In the second Cu site, Cu(2) is bonded to three equivalent Mg(1), two equivalent Yb(1), one Cu(1), one Cu(3), one Cu(5), and four equivalent Cu(2) atoms to form CuYb2Mg3Cu7 cuboctahedra that share corners with two equivalent Cu(5)Mg6Cu6 cuboctahedra, corners with six equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, corners with nine equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra,  a faceface with one Cu(6)Yb2Mg3Cu7 cuboctahedra, faces with three equivalent Cu(5)Mg6Cu6 cuboctahedra, and faces with ten equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra. The Cu(2)-Cu(3) bond length is 2.48 Å. The Cu(2)-Cu(5) bond length is 2.45 Å. There are two shorter (2.44 Å) and two longer (2.48 Å) Cu(2)-Cu(2) bond lengths. In the third Cu site, Cu(3) is bonded in a 12-coordinate geometry to three equivalent Yb(1), three equivalent Cu(1), three equivalent Cu(2), and three equivalent Cu(6) atoms. All Cu(3)-Cu(6) bond lengths are 2.48 Å. In the fourth Cu site, Cu(4) is bonded to six equivalent Mg(2) and six equivalent Cu(6) atoms to form CuMg6Cu6 cuboctahedra that share corners with twelve equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, edges with six equivalent Cu(4)Mg6Cu6 cuboctahedra, and faces with eighteen equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra. All Cu(4)-Cu(6) bond lengths are 2.44 Å. In the fifth Cu site, Cu(5) is bonded to six equivalent Mg(1) and six equivalent Cu(2) atoms to form CuMg6Cu6 cuboctahedra that share corners with twelve equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with six equivalent Cu(5)Mg6Cu6 cuboctahedra, and faces with eighteen equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra. In the sixth Cu site, Cu(6) is bonded to three equivalent Mg(2), two equivalent Yb(1), one Cu(1), one Cu(3), one Cu(4), and four equivalent Cu(6) atoms to form CuYb2Mg3Cu7 cuboctahedra that share corners with two equivalent Cu(4)Mg6Cu6 cuboctahedra, corners with six equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, corners with nine equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(2)Yb2Mg3Cu7 cuboctahedra, edges with four equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra,  a faceface with one Cu(2)Yb2Mg3Cu7 cuboctahedra, faces with three equivalent Cu(4)Mg6Cu6 cuboctahedra, and faces with ten equivalent Cu(6)Yb2Mg3Cu7 cuboctahedra. There are two shorter (2.44 Å) and two longer (2.48 Å) Cu(6)-Cu(6) bond lengths.