Mg8Ir23P8 crystallizes in the monoclinic P2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 11-coordinate geometry to one Ir(1), one Ir(2), one Ir(7), two equivalent Ir(10), two equivalent Ir(5), two equivalent Ir(9), two equivalent P(2), and two equivalent P(3) atoms. In the second Mg site, Mg(2) is bonded in a 11-coordinate geometry to one Ir(1), one Ir(2), one Ir(8), two equivalent Ir(10), two equivalent Ir(6), two equivalent Ir(9), two equivalent P(1), and two equivalent P(4) atoms. In the third Mg site, Mg(3) is bonded in a 13-coordinate geometry to one Ir(1), two equivalent Ir(10), two equivalent Ir(11), two equivalent Ir(3), two equivalent Ir(7), two equivalent P(1), and two equivalent P(3) atoms. In the fourth Mg site, Mg(4) is bonded in a 13-coordinate geometry to one Ir(2), two equivalent Ir(12), two equivalent Ir(4), two equivalent Ir(8), two equivalent Ir(9), two equivalent P(2), and two equivalent P(4) atoms. There are thirteen inequivalent Ir sites. In the first Ir site, Ir(1) is bonded in a 11-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), two equivalent Ir(3), two equivalent Ir(5), two equivalent Ir(9), and two equivalent P(1) atoms. In the second Ir site, Ir(2) is bonded in a 12-coordinate geometry to one Mg(1), one Mg(2), one Mg(4), one Ir(13), two equivalent Ir(10), two equivalent Ir(4), two equivalent Ir(6), and two equivalent P(2) atoms. In the third Ir site, Ir(3) is bonded in a 10-coordinate geometry to two equivalent Mg(3), one Ir(11), two equivalent Ir(1), two equivalent Ir(5), two equivalent Ir(7), and one P(1) atom. In the fourth Ir site, Ir(4) is bonded to two equivalent Mg(4), one Ir(12), two equivalent Ir(13), two equivalent Ir(2), two equivalent Ir(6), two equivalent Ir(8), and one P(2) atom to form distorted IrMg2PIr9 cuboctahedra that share corners with three equivalent Ir(4)Mg2PIr9 cuboctahedra, edges with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, edges with four equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra,  a faceface with one Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, and faces with three equivalent Ir(4)Mg2PIr9 cuboctahedra. In the fifth Ir site, Ir(5) is bonded in a 10-coordinate geometry to two equivalent Mg(1), one Ir(9), two equivalent Ir(1), two equivalent Ir(3), two equivalent Ir(7), and one P(3) atom. In the sixth Ir site, Ir(6) is bonded to two equivalent Mg(2), one Ir(10), two equivalent Ir(13), two equivalent Ir(2), two equivalent Ir(4), two equivalent Ir(8), and one P(4) atom to form distorted IrMg2PIr9 cuboctahedra that share corners with two equivalent Ir(6)Mg2PIr9 cuboctahedra, corners with three equivalent Ir(12)Mg4P2Ir6 cuboctahedra, edges with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra,  a faceface with one Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, and faces with three equivalent Ir(6)Mg2PIr9 cuboctahedra. In the seventh Ir site, Ir(7) is bonded in a 11-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Ir(11), two equivalent Ir(3), two equivalent Ir(5), and two equivalent P(3) atoms. In the eighth Ir site, Ir(8) is bonded to one Mg(2), two equivalent Mg(4), one Ir(13), two equivalent Ir(12), two equivalent Ir(4), two equivalent Ir(6), and two equivalent P(4) atoms to form distorted IrMg3P2Ir7 cuboctahedra that share corners with three equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with two equivalent Ir(13)Ir12 cuboctahedra, edges with two equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra,  a faceface with one Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, and faces with three equivalent Ir(8)Mg3P2Ir7 cuboctahedra. In the ninth Ir site, Ir(9) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), one Ir(5), two equivalent Ir(1), one P(1), one P(2), and one P(4) atom. In the tenth Ir site, Ir(10) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), one Ir(6), two equivalent Ir(2), one P(1), one P(2), and one P(3) atom. In the eleventh Ir site, Ir(11) is bonded to four equivalent Mg(3), two equivalent Ir(3), four equivalent Ir(7), and two equivalent P(3) atoms to form distorted face-sharing IrMg4P2Ir6 cuboctahedra. In the twelfth Ir site, Ir(12) is bonded to four equivalent Mg(4), two equivalent Ir(4), four equivalent Ir(8), and two equivalent P(4) atoms to form distorted IrMg4P2Ir6 cuboctahedra that share corners with six equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(13)Ir12 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, and faces with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra. In the thirteenth Ir site, Ir(13) is bonded to two equivalent Ir(2), two equivalent Ir(8), four equivalent Ir(4), and four equivalent Ir(6) atoms to form IrIr12 cuboctahedra that share edges with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with four equivalent Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, faces with four equivalent Ir(4)Mg2PIr9 cuboctahedra, and faces with four equivalent Ir(6)Mg2PIr9 cuboctahedra. There are four inequivalent P sites. In the first P site, P(1) is bonded in a 9-coordinate geometry to two equivalent Mg(2), two equivalent Mg(3), one Ir(10), one Ir(3), one Ir(9), and two equivalent Ir(1) atoms. In the second P site, P(2) is bonded in a 9-coordinate geometry to two equivalent Mg(1), two equivalent Mg(4), one Ir(10), one Ir(4), one Ir(9), and two equivalent Ir(2) atoms. In the third P site, P(3) is bonded in a 9-coordinate geometry to two equivalent Mg(1), two equivalent Mg(3), one Ir(10), one Ir(11), one Ir(5), and two equivalent Ir(7) atoms. In the fourth P site, P(4) is bonded in a 9-coordinate geometry to two equivalent Mg(2), two equivalent Mg(4), one Ir(12), one Ir(6), one Ir(9), and two equivalent Ir(8) atoms.
Mg8Ir23P8 crystallizes in the monoclinic P2/m space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 11-coordinate geometry to one Ir(1), one Ir(2), one Ir(7), two equivalent Ir(10), two equivalent Ir(5), two equivalent Ir(9), two equivalent P(2), and two equivalent P(3) atoms. The Mg(1)-Ir(1) bond length is 2.80 Å. The Mg(1)-Ir(2) bond length is 2.93 Å. The Mg(1)-Ir(7) bond length is 2.80 Å. Both Mg(1)-Ir(10) bond lengths are 3.25 Å. Both Mg(1)-Ir(5) bond lengths are 2.83 Å. Both Mg(1)-Ir(9) bond lengths are 2.86 Å. Both Mg(1)-P(2) bond lengths are 2.74 Å. Both Mg(1)-P(3) bond lengths are 2.83 Å. In the second Mg site, Mg(2) is bonded in a 11-coordinate geometry to one Ir(1), one Ir(2), one Ir(8), two equivalent Ir(10), two equivalent Ir(6), two equivalent Ir(9), two equivalent P(1), and two equivalent P(4) atoms. The Mg(2)-Ir(1) bond length is 2.88 Å. The Mg(2)-Ir(2) bond length is 2.82 Å. The Mg(2)-Ir(8) bond length is 2.86 Å. Both Mg(2)-Ir(10) bond lengths are 2.85 Å. Both Mg(2)-Ir(6) bond lengths are 2.85 Å. Both Mg(2)-Ir(9) bond lengths are 3.23 Å. Both Mg(2)-P(1) bond lengths are 2.72 Å. Both Mg(2)-P(4) bond lengths are 2.84 Å. In the third Mg site, Mg(3) is bonded in a 13-coordinate geometry to one Ir(1), two equivalent Ir(10), two equivalent Ir(11), two equivalent Ir(3), two equivalent Ir(7), two equivalent P(1), and two equivalent P(3) atoms. The Mg(3)-Ir(1) bond length is 2.77 Å. Both Mg(3)-Ir(10) bond lengths are 3.10 Å. Both Mg(3)-Ir(11) bond lengths are 2.82 Å. Both Mg(3)-Ir(3) bond lengths are 2.91 Å. There is one shorter (2.82 Å) and one longer (2.83 Å) Mg(3)-Ir(7) bond length. Both Mg(3)-P(1) bond lengths are 2.79 Å. Both Mg(3)-P(3) bond lengths are 2.80 Å. In the fourth Mg site, Mg(4) is bonded in a 13-coordinate geometry to one Ir(2), two equivalent Ir(12), two equivalent Ir(4), two equivalent Ir(8), two equivalent Ir(9), two equivalent P(2), and two equivalent P(4) atoms. The Mg(4)-Ir(2) bond length is 2.77 Å. Both Mg(4)-Ir(12) bond lengths are 2.85 Å. Both Mg(4)-Ir(4) bond lengths are 2.88 Å. There is one shorter (2.77 Å) and one longer (2.84 Å) Mg(4)-Ir(8) bond length. Both Mg(4)-Ir(9) bond lengths are 3.08 Å. Both Mg(4)-P(2) bond lengths are 2.78 Å. Both Mg(4)-P(4) bond lengths are 2.79 Å. There are thirteen inequivalent Ir sites. In the first Ir site, Ir(1) is bonded in a 11-coordinate geometry to one Mg(1), one Mg(2), one Mg(3), two equivalent Ir(3), two equivalent Ir(5), two equivalent Ir(9), and two equivalent P(1) atoms. Both Ir(1)-Ir(3) bond lengths are 2.78 Å. Both Ir(1)-Ir(5) bond lengths are 2.73 Å. Both Ir(1)-Ir(9) bond lengths are 2.86 Å. Both Ir(1)-P(1) bond lengths are 2.41 Å. In the second Ir site, Ir(2) is bonded in a 12-coordinate geometry to one Mg(1), one Mg(2), one Mg(4), one Ir(13), two equivalent Ir(10), two equivalent Ir(4), two equivalent Ir(6), and two equivalent P(2) atoms. The Ir(2)-Ir(13) bond length is 2.74 Å. Both Ir(2)-Ir(10) bond lengths are 2.86 Å. Both Ir(2)-Ir(4) bond lengths are 2.79 Å. Both Ir(2)-Ir(6) bond lengths are 2.75 Å. Both Ir(2)-P(2) bond lengths are 2.43 Å. In the third Ir site, Ir(3) is bonded in a 10-coordinate geometry to two equivalent Mg(3), one Ir(11), two equivalent Ir(1), two equivalent Ir(5), two equivalent Ir(7), and one P(1) atom. The Ir(3)-Ir(11) bond length is 2.79 Å. There is one shorter (2.63 Å) and one longer (2.73 Å) Ir(3)-Ir(5) bond length. Both Ir(3)-Ir(7) bond lengths are 2.71 Å. The Ir(3)-P(1) bond length is 2.37 Å. In the fourth Ir site, Ir(4) is bonded to two equivalent Mg(4), one Ir(12), two equivalent Ir(13), two equivalent Ir(2), two equivalent Ir(6), two equivalent Ir(8), and one P(2) atom to form distorted IrMg2PIr9 cuboctahedra that share corners with three equivalent Ir(4)Mg2PIr9 cuboctahedra, edges with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, edges with four equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra,  a faceface with one Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, and faces with three equivalent Ir(4)Mg2PIr9 cuboctahedra. The Ir(4)-Ir(12) bond length is 2.76 Å. Both Ir(4)-Ir(13) bond lengths are 2.76 Å. There is one shorter (2.72 Å) and one longer (2.76 Å) Ir(4)-Ir(6) bond length. Both Ir(4)-Ir(8) bond lengths are 2.75 Å. The Ir(4)-P(2) bond length is 2.36 Å. In the fifth Ir site, Ir(5) is bonded in a 10-coordinate geometry to two equivalent Mg(1), one Ir(9), two equivalent Ir(1), two equivalent Ir(3), two equivalent Ir(7), and one P(3) atom. The Ir(5)-Ir(9) bond length is 2.82 Å. Both Ir(5)-Ir(7) bond lengths are 2.77 Å. The Ir(5)-P(3) bond length is 2.36 Å. In the sixth Ir site, Ir(6) is bonded to two equivalent Mg(2), one Ir(10), two equivalent Ir(13), two equivalent Ir(2), two equivalent Ir(4), two equivalent Ir(8), and one P(4) atom to form distorted IrMg2PIr9 cuboctahedra that share corners with two equivalent Ir(6)Mg2PIr9 cuboctahedra, corners with three equivalent Ir(12)Mg4P2Ir6 cuboctahedra, edges with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra,  a faceface with one Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, and faces with three equivalent Ir(6)Mg2PIr9 cuboctahedra. The Ir(6)-Ir(10) bond length is 2.84 Å. Both Ir(6)-Ir(13) bond lengths are 2.81 Å. Both Ir(6)-Ir(8) bond lengths are 2.78 Å. The Ir(6)-P(4) bond length is 2.32 Å. In the seventh Ir site, Ir(7) is bonded in a 11-coordinate geometry to one Mg(1), two equivalent Mg(3), two equivalent Ir(11), two equivalent Ir(3), two equivalent Ir(5), and two equivalent P(3) atoms. Both Ir(7)-Ir(11) bond lengths are 2.83 Å. Both Ir(7)-P(3) bond lengths are 2.45 Å. In the eighth Ir site, Ir(8) is bonded to one Mg(2), two equivalent Mg(4), one Ir(13), two equivalent Ir(12), two equivalent Ir(4), two equivalent Ir(6), and two equivalent P(4) atoms to form distorted IrMg3P2Ir7 cuboctahedra that share corners with three equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with two equivalent Ir(13)Ir12 cuboctahedra, edges with two equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra,  a faceface with one Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, and faces with three equivalent Ir(8)Mg3P2Ir7 cuboctahedra. The Ir(8)-Ir(13) bond length is 2.67 Å. Both Ir(8)-Ir(12) bond lengths are 2.78 Å. Both Ir(8)-P(4) bond lengths are 2.45 Å. In the ninth Ir site, Ir(9) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(4), one Ir(5), two equivalent Ir(1), one P(1), one P(2), and one P(4) atom. The Ir(9)-P(1) bond length is 2.45 Å. The Ir(9)-P(2) bond length is 2.38 Å. The Ir(9)-P(4) bond length is 2.29 Å. In the tenth Ir site, Ir(10) is bonded in a 12-coordinate geometry to two equivalent Mg(1), two equivalent Mg(2), two equivalent Mg(3), one Ir(6), two equivalent Ir(2), one P(1), one P(2), and one P(3) atom. The Ir(10)-P(1) bond length is 2.40 Å. The Ir(10)-P(2) bond length is 2.46 Å. The Ir(10)-P(3) bond length is 2.31 Å. In the eleventh Ir site, Ir(11) is bonded to four equivalent Mg(3), two equivalent Ir(3), four equivalent Ir(7), and two equivalent P(3) atoms to form distorted face-sharing IrMg4P2Ir6 cuboctahedra. Both Ir(11)-P(3) bond lengths are 2.34 Å. In the twelfth Ir site, Ir(12) is bonded to four equivalent Mg(4), two equivalent Ir(4), four equivalent Ir(8), and two equivalent P(4) atoms to form distorted IrMg4P2Ir6 cuboctahedra that share corners with six equivalent Ir(6)Mg2PIr9 cuboctahedra, edges with four equivalent Ir(13)Ir12 cuboctahedra, edges with four equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(4)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(6)Mg2PIr9 cuboctahedra, faces with two equivalent Ir(12)Mg4P2Ir6 cuboctahedra, and faces with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra. Both Ir(12)-P(4) bond lengths are 2.37 Å. In the thirteenth Ir site, Ir(13) is bonded to two equivalent Ir(2), two equivalent Ir(8), four equivalent Ir(4), and four equivalent Ir(6) atoms to form IrIr12 cuboctahedra that share edges with four equivalent Ir(8)Mg3P2Ir7 cuboctahedra, edges with four equivalent Ir(12)Mg4P2Ir6 cuboctahedra, faces with two equivalent Ir(13)Ir12 cuboctahedra, faces with two equivalent Ir(8)Mg3P2Ir7 cuboctahedra, faces with four equivalent Ir(4)Mg2PIr9 cuboctahedra, and faces with four equivalent Ir(6)Mg2PIr9 cuboctahedra. There are four inequivalent P sites. In the first P site, P(1) is bonded in a 9-coordinate geometry to two equivalent Mg(2), two equivalent Mg(3), one Ir(10), one Ir(3), one Ir(9), and two equivalent Ir(1) atoms. In the second P site, P(2) is bonded in a 9-coordinate geometry to two equivalent Mg(1), two equivalent Mg(4), one Ir(10), one Ir(4), one Ir(9), and two equivalent Ir(2) atoms. In the third P site, P(3) is bonded in a 9-coordinate geometry to two equivalent Mg(1), two equivalent Mg(3), one Ir(10), one Ir(11), one Ir(5), and two equivalent Ir(7) atoms. In the fourth P site, P(4) is bonded in a 9-coordinate geometry to two equivalent Mg(2), two equivalent Mg(4), one Ir(12), one Ir(6), one Ir(9), and two equivalent Ir(8) atoms.