MgPt5 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one Pt(3), two equivalent Pt(1), two equivalent Pt(2), two equivalent Pt(4), and three equivalent Pt(5) atoms to form distorted MgPt10 cuboctahedra that share corners with two equivalent Pt(1)Mg2Pt10 cuboctahedra, corners with two equivalent Pt(5)Mg3Pt2 cuboctahedra, corners with four equivalent Mg(1)Pt10 cuboctahedra,  an edgeedge with one Pt(5)Mg3Pt2 cuboctahedra, edges with four equivalent Pt(1)Mg2Pt10 cuboctahedra, faces with two equivalent Mg(1)Pt10 cuboctahedra, and faces with three equivalent Pt(1)Mg2Pt10 cuboctahedra. There are five inequivalent Pt sites. In the first Pt site, Pt(1) is bonded to two equivalent Mg(1), one Pt(4), two equivalent Pt(1), two equivalent Pt(3), two equivalent Pt(5), and three equivalent Pt(2) atoms to form distorted PtMg2Pt10 cuboctahedra that share corners with two equivalent Mg(1)Pt10 cuboctahedra, corners with four equivalent Pt(5)Mg3Pt2 cuboctahedra, corners with six equivalent Pt(1)Mg2Pt10 cuboctahedra, edges with three equivalent Pt(5)Mg3Pt2 cuboctahedra, edges with four equivalent Mg(1)Pt10 cuboctahedra, faces with two equivalent Pt(1)Mg2Pt10 cuboctahedra, and faces with three equivalent Mg(1)Pt10 cuboctahedra. In the second Pt site, Pt(2) is bonded in a 2-coordinate geometry to two equivalent Mg(1) and three equivalent Pt(1) atoms. In the third Pt site, Pt(3) is bonded in a distorted single-bond geometry to one Mg(1) and two equivalent Pt(1) atoms. In the fourth Pt site, Pt(4) is bonded in a 2-coordinate geometry to two equivalent Mg(1) and one Pt(1) atom. In the fifth Pt site, Pt(5) is bonded to three equivalent Mg(1) and two equivalent Pt(1) atoms to form distorted PtMg3Pt2 cuboctahedra that share corners with two equivalent Mg(1)Pt10 cuboctahedra, corners with four equivalent Pt(1)Mg2Pt10 cuboctahedra, corners with four equivalent Pt(5)Mg3Pt2 cuboctahedra,  an edgeedge with one Mg(1)Pt10 cuboctahedra, edges with two equivalent Pt(5)Mg3Pt2 cuboctahedra, and edges with three equivalent Pt(1)Mg2Pt10 cuboctahedra.
MgPt5 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one Pt(3), two equivalent Pt(1), two equivalent Pt(2), two equivalent Pt(4), and three equivalent Pt(5) atoms to form distorted MgPt10 cuboctahedra that share corners with two equivalent Pt(1)Mg2Pt10 cuboctahedra, corners with two equivalent Pt(5)Mg3Pt2 cuboctahedra, corners with four equivalent Mg(1)Pt10 cuboctahedra,  an edgeedge with one Pt(5)Mg3Pt2 cuboctahedra, edges with four equivalent Pt(1)Mg2Pt10 cuboctahedra, faces with two equivalent Mg(1)Pt10 cuboctahedra, and faces with three equivalent Pt(1)Mg2Pt10 cuboctahedra. The Mg(1)-Pt(3) bond length is 2.80 Å. There is one shorter (2.82 Å) and one longer (2.83 Å) Mg(1)-Pt(1) bond length. There is one shorter (2.68 Å) and one longer (2.70 Å) Mg(1)-Pt(2) bond length. There is one shorter (2.71 Å) and one longer (2.72 Å) Mg(1)-Pt(4) bond length. There are a spread of Mg(1)-Pt(5) bond distances ranging from 2.79-2.81 Å. There are five inequivalent Pt sites. In the first Pt site, Pt(1) is bonded to two equivalent Mg(1), one Pt(4), two equivalent Pt(1), two equivalent Pt(3), two equivalent Pt(5), and three equivalent Pt(2) atoms to form distorted PtMg2Pt10 cuboctahedra that share corners with two equivalent Mg(1)Pt10 cuboctahedra, corners with four equivalent Pt(5)Mg3Pt2 cuboctahedra, corners with six equivalent Pt(1)Mg2Pt10 cuboctahedra, edges with three equivalent Pt(5)Mg3Pt2 cuboctahedra, edges with four equivalent Mg(1)Pt10 cuboctahedra, faces with two equivalent Pt(1)Mg2Pt10 cuboctahedra, and faces with three equivalent Mg(1)Pt10 cuboctahedra. The Pt(1)-Pt(4) bond length is 2.85 Å. Both Pt(1)-Pt(1) bond lengths are 2.77 Å. There is one shorter (2.75 Å) and one longer (2.76 Å) Pt(1)-Pt(3) bond length. There is one shorter (2.75 Å) and one longer (2.76 Å) Pt(1)-Pt(5) bond length. There are a spread of Pt(1)-Pt(2) bond distances ranging from 2.74-2.85 Å. In the second Pt site, Pt(2) is bonded in a 2-coordinate geometry to two equivalent Mg(1) and three equivalent Pt(1) atoms. In the third Pt site, Pt(3) is bonded in a distorted single-bond geometry to one Mg(1) and two equivalent Pt(1) atoms. In the fourth Pt site, Pt(4) is bonded in a 2-coordinate geometry to two equivalent Mg(1) and one Pt(1) atom. In the fifth Pt site, Pt(5) is bonded to three equivalent Mg(1) and two equivalent Pt(1) atoms to form distorted PtMg3Pt2 cuboctahedra that share corners with two equivalent Mg(1)Pt10 cuboctahedra, corners with four equivalent Pt(1)Mg2Pt10 cuboctahedra, corners with four equivalent Pt(5)Mg3Pt2 cuboctahedra,  an edgeedge with one Mg(1)Pt10 cuboctahedra, edges with two equivalent Pt(5)Mg3Pt2 cuboctahedra, and edges with three equivalent Pt(1)Mg2Pt10 cuboctahedra.