Mg4B8H31 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one H(10), one H(19), one H(2), one H(23), one H(24), one H(25), one H(29), and one H(4) atom to form distorted MgH8 hexagonal bipyramids that share  an edgeedge with one B(4)H4 tetrahedra,  an edgeedge with one B(5)H4 tetrahedra,  an edgeedge with one B(6)H4 tetrahedra, and  an edgeedge with one B(7)H4 tetrahedra. In the second Mg site, Mg(2) is bonded in a 8-coordinate geometry to one B(2), one H(15), one H(21), one H(30), one H(31), one H(7), one H(8), and one H(9) atom. In the third Mg site, Mg(3) is bonded in a distorted body-centered cubic geometry to one H(1), one H(11), one H(16), one H(17), one H(18), one H(20), one H(22), and one H(3) atom. In the fourth Mg site, Mg(4) is bonded in a body-centered cubic geometry to one H(12), one H(13), one H(14), one H(26), one H(27), one H(28), one H(5), and one H(6) atom. There are eight inequivalent B sites. In the first B site, B(1) is bonded in a tetrahedral geometry to one H(14), one H(27), one H(30), and one H(7) atom. In the second B site, B(2) is bonded in a trigonal planar geometry to one Mg(2), one H(16), one H(20), and one H(9) atom. In the third B site, B(3) is bonded in a tetrahedral geometry to one H(1), one H(17), one H(21), and one H(8) atom. In the fourth B site, B(4) is bonded to one H(11), one H(18), one H(2), and one H(23) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. In the fifth B site, B(5) is bonded to one H(10), one H(19), one H(22), and one H(3) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. In the sixth B site, B(6) is bonded to one H(13), one H(24), one H(28), and one H(4) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. In the seventh B site, B(7) is bonded to one H(12), one H(25), one H(29), and one H(5) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. In the eighth B site, B(8) is bonded in a tetrahedral geometry to one H(15), one H(26), one H(31), and one H(6) atom. There are thirty-one inequivalent H sites. In the first H site, H(1) is bonded in an L-shaped geometry to one Mg(3) and one B(3) atom. In the second H site, H(2) is bonded in an L-shaped geometry to one Mg(1) and one B(4) atom. In the third H site, H(3) is bonded in an L-shaped geometry to one Mg(3) and one B(5) atom. In the fourth H site, H(4) is bonded in an L-shaped geometry to one Mg(1) and one B(6) atom. In the fifth H site, H(5) is bonded in an L-shaped geometry to one Mg(4) and one B(7) atom. In the sixth H site, H(6) is bonded in an L-shaped geometry to one Mg(4) and one B(8) atom. In the seventh H site, H(7) is bonded in an L-shaped geometry to one Mg(2) and one B(1) atom. In the eighth H site, H(8) is bonded in an L-shaped geometry to one Mg(2) and one B(3) atom. In the ninth H site, H(9) is bonded in a distorted L-shaped geometry to one Mg(2) and one B(2) atom. In the tenth H site, H(10) is bonded in an L-shaped geometry to one Mg(1) and one B(5) atom. In the eleventh H site, H(11) is bonded in an L-shaped geometry to one Mg(3) and one B(4) atom. In the twelfth H site, H(12) is bonded in an L-shaped geometry to one Mg(4) and one B(7) atom. In the thirteenth H site, H(13) is bonded in an L-shaped geometry to one Mg(4) and one B(6) atom. In the fourteenth H site, H(14) is bonded in an L-shaped geometry to one Mg(4) and one B(1) atom. In the fifteenth H site, H(15) is bonded in an L-shaped geometry to one Mg(2) and one B(8) atom. In the sixteenth H site, H(16) is bonded in a distorted L-shaped geometry to one Mg(3) and one B(2) atom. In the seventeenth H site, H(17) is bonded in an L-shaped geometry to one Mg(3) and one B(3) atom. In the eighteenth H site, H(18) is bonded in an L-shaped geometry to one Mg(3) and one B(4) atom. In the nineteenth H site, H(19) is bonded in an L-shaped geometry to one Mg(1) and one B(5) atom. In the twentieth H site, H(20) is bonded in a distorted L-shaped geometry to one Mg(3) and one B(2) atom. In the twenty-first H site, H(21) is bonded in an L-shaped geometry to one Mg(2) and one B(3) atom. In the twenty-second H site, H(22) is bonded in an L-shaped geometry to one Mg(3) and one B(5) atom. In the twenty-third H site, H(23) is bonded in an L-shaped geometry to one Mg(1) and one B(4) atom. In the twenty-fourth H site, H(24) is bonded in an L-shaped geometry to one Mg(1) and one B(6) atom. In the twenty-fifth H site, H(25) is bonded in an L-shaped geometry to one Mg(1) and one B(7) atom. In the twenty-sixth H site, H(26) is bonded in an L-shaped geometry to one Mg(4) and one B(8) atom. In the twenty-seventh H site, H(27) is bonded in an L-shaped geometry to one Mg(4) and one B(1) atom. In the twenty-eighth H site, H(28) is bonded in an L-shaped geometry to one Mg(4) and one B(6) atom. In the twenty-ninth H site, H(29) is bonded in an L-shaped geometry to one Mg(1) and one B(7) atom. In the thirtieth H site, H(30) is bonded in an L-shaped geometry to one Mg(2) and one B(1) atom. In the thirty-first H site, H(31) is bonded in an L-shaped geometry to one Mg(2) and one B(8) atom.
Mg4B8H31 crystallizes in the triclinic P1 space group. There are four inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one H(10), one H(19), one H(2), one H(23), one H(24), one H(25), one H(29), and one H(4) atom to form distorted MgH8 hexagonal bipyramids that share  an edgeedge with one B(4)H4 tetrahedra,  an edgeedge with one B(5)H4 tetrahedra,  an edgeedge with one B(6)H4 tetrahedra, and  an edgeedge with one B(7)H4 tetrahedra. The Mg(1)-H(10) bond length is 2.08 Å. The Mg(1)-H(19) bond length is 2.05 Å. The Mg(1)-H(2) bond length is 2.10 Å. The Mg(1)-H(23) bond length is 2.04 Å. The Mg(1)-H(24) bond length is 2.04 Å. The Mg(1)-H(25) bond length is 2.08 Å. The Mg(1)-H(29) bond length is 2.04 Å. The Mg(1)-H(4) bond length is 2.08 Å. In the second Mg site, Mg(2) is bonded in a 8-coordinate geometry to one B(2), one H(15), one H(21), one H(30), one H(31), one H(7), one H(8), and one H(9) atom. The Mg(2)-B(2) bond length is 2.47 Å. The Mg(2)-H(15) bond length is 2.09 Å. The Mg(2)-H(21) bond length is 2.03 Å. The Mg(2)-H(30) bond length is 2.11 Å. The Mg(2)-H(31) bond length is 2.05 Å. The Mg(2)-H(7) bond length is 2.04 Å. The Mg(2)-H(8) bond length is 2.10 Å. The Mg(2)-H(9) bond length is 2.24 Å. In the third Mg site, Mg(3) is bonded in a distorted body-centered cubic geometry to one H(1), one H(11), one H(16), one H(17), one H(18), one H(20), one H(22), and one H(3) atom. The Mg(3)-H(1) bond length is 2.09 Å. The Mg(3)-H(11) bond length is 2.08 Å. The Mg(3)-H(16) bond length is 2.24 Å. The Mg(3)-H(17) bond length is 2.09 Å. The Mg(3)-H(18) bond length is 2.09 Å. The Mg(3)-H(20) bond length is 2.23 Å. The Mg(3)-H(22) bond length is 2.07 Å. The Mg(3)-H(3) bond length is 2.09 Å. In the fourth Mg site, Mg(4) is bonded in a body-centered cubic geometry to one H(12), one H(13), one H(14), one H(26), one H(27), one H(28), one H(5), and one H(6) atom. The Mg(4)-H(12) bond length is 2.08 Å. The Mg(4)-H(13) bond length is 2.08 Å. The Mg(4)-H(14) bond length is 2.09 Å. The Mg(4)-H(26) bond length is 2.08 Å. The Mg(4)-H(27) bond length is 2.09 Å. The Mg(4)-H(28) bond length is 2.08 Å. The Mg(4)-H(5) bond length is 2.09 Å. The Mg(4)-H(6) bond length is 2.08 Å. There are eight inequivalent B sites. In the first B site, B(1) is bonded in a tetrahedral geometry to one H(14), one H(27), one H(30), and one H(7) atom. The B(1)-H(14) bond length is 1.22 Å. The B(1)-H(27) bond length is 1.22 Å. The B(1)-H(30) bond length is 1.23 Å. The B(1)-H(7) bond length is 1.23 Å. In the second B site, B(2) is bonded in a trigonal planar geometry to one Mg(2), one H(16), one H(20), and one H(9) atom. The B(2)-H(16) bond length is 1.21 Å. The B(2)-H(20) bond length is 1.21 Å. The B(2)-H(9) bond length is 1.22 Å. In the third B site, B(3) is bonded in a tetrahedral geometry to one H(1), one H(17), one H(21), and one H(8) atom. The B(3)-H(1) bond length is 1.22 Å. The B(3)-H(17) bond length is 1.22 Å. The B(3)-H(21) bond length is 1.23 Å. The B(3)-H(8) bond length is 1.23 Å. In the fourth B site, B(4) is bonded to one H(11), one H(18), one H(2), and one H(23) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. The B(4)-H(11) bond length is 1.22 Å. The B(4)-H(18) bond length is 1.22 Å. The B(4)-H(2) bond length is 1.23 Å. The B(4)-H(23) bond length is 1.23 Å. In the fifth B site, B(5) is bonded to one H(10), one H(19), one H(22), and one H(3) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. The B(5)-H(10) bond length is 1.23 Å. The B(5)-H(19) bond length is 1.23 Å. The B(5)-H(22) bond length is 1.22 Å. The B(5)-H(3) bond length is 1.22 Å. In the sixth B site, B(6) is bonded to one H(13), one H(24), one H(28), and one H(4) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. The B(6)-H(13) bond length is 1.22 Å. The B(6)-H(24) bond length is 1.23 Å. The B(6)-H(28) bond length is 1.22 Å. The B(6)-H(4) bond length is 1.23 Å. In the seventh B site, B(7) is bonded to one H(12), one H(25), one H(29), and one H(5) atom to form BH4 tetrahedra that share  an edgeedge with one Mg(1)H8 hexagonal bipyramid. The B(7)-H(12) bond length is 1.22 Å. The B(7)-H(25) bond length is 1.23 Å. The B(7)-H(29) bond length is 1.22 Å. The B(7)-H(5) bond length is 1.22 Å. In the eighth B site, B(8) is bonded in a tetrahedral geometry to one H(15), one H(26), one H(31), and one H(6) atom. The B(8)-H(15) bond length is 1.23 Å. The B(8)-H(26) bond length is 1.22 Å. The B(8)-H(31) bond length is 1.23 Å. The B(8)-H(6) bond length is 1.22 Å. There are thirty-one inequivalent H sites. In the first H site, H(1) is bonded in an L-shaped geometry to one Mg(3) and one B(3) atom. In the second H site, H(2) is bonded in an L-shaped geometry to one Mg(1) and one B(4) atom. In the third H site, H(3) is bonded in an L-shaped geometry to one Mg(3) and one B(5) atom. In the fourth H site, H(4) is bonded in an L-shaped geometry to one Mg(1) and one B(6) atom. In the fifth H site, H(5) is bonded in an L-shaped geometry to one Mg(4) and one B(7) atom. In the sixth H site, H(6) is bonded in an L-shaped geometry to one Mg(4) and one B(8) atom. In the seventh H site, H(7) is bonded in an L-shaped geometry to one Mg(2) and one B(1) atom. In the eighth H site, H(8) is bonded in an L-shaped geometry to one Mg(2) and one B(3) atom. In the ninth H site, H(9) is bonded in a distorted L-shaped geometry to one Mg(2) and one B(2) atom. In the tenth H site, H(10) is bonded in an L-shaped geometry to one Mg(1) and one B(5) atom. In the eleventh H site, H(11) is bonded in an L-shaped geometry to one Mg(3) and one B(4) atom. In the twelfth H site, H(12) is bonded in an L-shaped geometry to one Mg(4) and one B(7) atom. In the thirteenth H site, H(13) is bonded in an L-shaped geometry to one Mg(4) and one B(6) atom. In the fourteenth H site, H(14) is bonded in an L-shaped geometry to one Mg(4) and one B(1) atom. In the fifteenth H site, H(15) is bonded in an L-shaped geometry to one Mg(2) and one B(8) atom. In the sixteenth H site, H(16) is bonded in a distorted L-shaped geometry to one Mg(3) and one B(2) atom. In the seventeenth H site, H(17) is bonded in an L-shaped geometry to one Mg(3) and one B(3) atom. In the eighteenth H site, H(18) is bonded in an L-shaped geometry to one Mg(3) and one B(4) atom. In the nineteenth H site, H(19) is bonded in an L-shaped geometry to one Mg(1) and one B(5) atom. In the twentieth H site, H(20) is bonded in a distorted L-shaped geometry to one Mg(3) and one B(2) atom. In the twenty-first H site, H(21) is bonded in an L-shaped geometry to one Mg(2) and one B(3) atom. In the twenty-second H site, H(22) is bonded in an L-shaped geometry to one Mg(3) and one B(5) atom. In the twenty-third H site, H(23) is bonded in an L-shaped geometry to one Mg(1) and one B(4) atom. In the twenty-fourth H site, H(24) is bonded in an L-shaped geometry to one Mg(1) and one B(6) atom. In the twenty-fifth H site, H(25) is bonded in an L-shaped geometry to one Mg(1) and one B(7) atom. In the twenty-sixth H site, H(26) is bonded in an L-shaped geometry to one Mg(4) and one B(8) atom. In the twenty-seventh H site, H(27) is bonded in an L-shaped geometry to one Mg(4) and one B(1) atom. In the twenty-eighth H site, H(28) is bonded in an L-shaped geometry to one Mg(4) and one B(6) atom. In the twenty-ninth H site, H(29) is bonded in an L-shaped geometry to one Mg(1) and one B(7) atom. In the thirtieth H site, H(30) is bonded in an L-shaped geometry to one Mg(2) and one B(1) atom. In the thirty-first H site, H(31) is bonded in an L-shaped geometry to one Mg(2) and one B(8) atom.