TbB18Si4C crystallizes in the monoclinic C2/m space group. Tb(1) is bonded in a 18-coordinate geometry to two equivalent B(10), two equivalent B(3), two equivalent B(4), two equivalent B(6), two equivalent B(7), two equivalent B(9), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms. There are ten inequivalent B sites. In the first B site, B(5) is bonded in a 6-coordinate geometry to one B(1), one B(10), one B(3), one B(6), one B(8), and one Si(2) atom. In the second B site, B(6) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(10), one B(3), one B(5), one B(8), and one Si(3) atom. In the third B site, B(7) is bonded in a 7-coordinate geometry to one Tb(1), one B(2), one B(4), one B(7), two equivalent B(9), and one Si(2) atom. In the fourth B site, B(8) is bonded in a 6-coordinate geometry to one B(1), one B(10), one B(3), one B(5), one B(6), and one B(8) atom. In the fifth B site, B(9) is bonded in a 7-coordinate geometry to one Tb(1), one B(10), one B(2), one B(4), one B(9), and two equivalent B(7) atoms. In the sixth B site, B(10) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(3), one B(5), one B(6), one B(8), and one B(9) atom. In the seventh B site, B(1) is bonded in a distorted single-bond geometry to one B(10), one B(3), one B(5), one B(6), one B(8), and one C(1) atom. In the eighth B site, B(2) is bonded in a single-bond geometry to one B(4), two equivalent B(7), two equivalent B(9), and one C(1) atom. In the ninth B site, B(3) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(10), one B(5), one B(6), one B(8), and one Si(2) atom. In the tenth B site, B(4) is bonded in a 8-coordinate geometry to two equivalent Tb(1), one B(2), two equivalent B(7), two equivalent B(9), and one Si(3) atom. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 6-coordinate geometry to two equivalent Tb(1), one Si(1), one Si(3), and two equivalent Si(2) atoms. In the second Si site, Si(2) is bonded in a 5-coordinate geometry to one Tb(1), one B(3), one B(5), one B(7), and one Si(1) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to two equivalent Tb(1), one B(4), two equivalent B(6), and one Si(1) atom. C(1) is bonded in a distorted tetrahedral geometry to one B(2), two equivalent B(1), and one C(1) atom.
TbB18Si4C crystallizes in the monoclinic C2/m space group. Tb(1) is bonded in a 18-coordinate geometry to two equivalent B(10), two equivalent B(3), two equivalent B(4), two equivalent B(6), two equivalent B(7), two equivalent B(9), two equivalent Si(1), two equivalent Si(2), and two equivalent Si(3) atoms. Both Tb(1)-B(10) bond lengths are 2.75 Å. Both Tb(1)-B(3) bond lengths are 2.91 Å. Both Tb(1)-B(4) bond lengths are 2.92 Å. Both Tb(1)-B(6) bond lengths are 2.80 Å. Both Tb(1)-B(7) bond lengths are 2.78 Å. Both Tb(1)-B(9) bond lengths are 2.75 Å. Both Tb(1)-Si(1) bond lengths are 3.28 Å. Both Tb(1)-Si(2) bond lengths are 2.87 Å. Both Tb(1)-Si(3) bond lengths are 2.84 Å. There are ten inequivalent B sites. In the first B site, B(5) is bonded in a 6-coordinate geometry to one B(1), one B(10), one B(3), one B(6), one B(8), and one Si(2) atom. The B(5)-B(1) bond length is 1.81 Å. The B(5)-B(10) bond length is 1.80 Å. The B(5)-B(3) bond length is 1.76 Å. The B(5)-B(6) bond length is 1.77 Å. The B(5)-B(8) bond length is 1.78 Å. The B(5)-Si(2) bond length is 1.99 Å. In the second B site, B(6) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(10), one B(3), one B(5), one B(8), and one Si(3) atom. The B(6)-B(1) bond length is 1.80 Å. The B(6)-B(10) bond length is 1.78 Å. The B(6)-B(3) bond length is 1.77 Å. The B(6)-B(8) bond length is 1.79 Å. The B(6)-Si(3) bond length is 2.02 Å. In the third B site, B(7) is bonded in a 7-coordinate geometry to one Tb(1), one B(2), one B(4), one B(7), two equivalent B(9), and one Si(2) atom. The B(7)-B(2) bond length is 1.83 Å. The B(7)-B(4) bond length is 1.79 Å. The B(7)-B(7) bond length is 1.79 Å. There is one shorter (1.79 Å) and one longer (1.80 Å) B(7)-B(9) bond length. The B(7)-Si(2) bond length is 2.07 Å. In the fourth B site, B(8) is bonded in a 6-coordinate geometry to one B(1), one B(10), one B(3), one B(5), one B(6), and one B(8) atom. The B(8)-B(1) bond length is 1.86 Å. The B(8)-B(10) bond length is 1.79 Å. The B(8)-B(3) bond length is 1.79 Å. The B(8)-B(8) bond length is 1.71 Å. In the fifth B site, B(9) is bonded in a 7-coordinate geometry to one Tb(1), one B(10), one B(2), one B(4), one B(9), and two equivalent B(7) atoms. The B(9)-B(10) bond length is 1.74 Å. The B(9)-B(2) bond length is 1.84 Å. The B(9)-B(4) bond length is 1.80 Å. The B(9)-B(9) bond length is 1.79 Å. In the sixth B site, B(10) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(3), one B(5), one B(6), one B(8), and one B(9) atom. The B(10)-B(1) bond length is 1.82 Å. The B(10)-B(3) bond length is 1.79 Å. In the seventh B site, B(1) is bonded in a distorted single-bond geometry to one B(10), one B(3), one B(5), one B(6), one B(8), and one C(1) atom. The B(1)-B(3) bond length is 1.85 Å. The B(1)-C(1) bond length is 1.69 Å. In the eighth B site, B(2) is bonded in a single-bond geometry to one B(4), two equivalent B(7), two equivalent B(9), and one C(1) atom. The B(2)-B(4) bond length is 1.85 Å. The B(2)-C(1) bond length is 1.67 Å. In the ninth B site, B(3) is bonded in a 7-coordinate geometry to one Tb(1), one B(1), one B(10), one B(5), one B(6), one B(8), and one Si(2) atom. The B(3)-Si(2) bond length is 2.02 Å. In the tenth B site, B(4) is bonded in a 8-coordinate geometry to two equivalent Tb(1), one B(2), two equivalent B(7), two equivalent B(9), and one Si(3) atom. The B(4)-Si(3) bond length is 2.08 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 6-coordinate geometry to two equivalent Tb(1), one Si(1), one Si(3), and two equivalent Si(2) atoms. The Si(1)-Si(1) bond length is 2.33 Å. The Si(1)-Si(3) bond length is 2.36 Å. Both Si(1)-Si(2) bond lengths are 2.34 Å. In the second Si site, Si(2) is bonded in a 5-coordinate geometry to one Tb(1), one B(3), one B(5), one B(7), and one Si(1) atom. In the third Si site, Si(3) is bonded in a 6-coordinate geometry to two equivalent Tb(1), one B(4), two equivalent B(6), and one Si(1) atom. C(1) is bonded in a distorted tetrahedral geometry to one B(2), two equivalent B(1), and one C(1) atom. The C(1)-C(1) bond length is 1.66 Å.