Sm2Co18Si4C3 crystallizes in the monoclinic C2/m space group. Sm(1) is bonded in a 3-coordinate geometry to two equivalent Co(5), four equivalent Co(3), one C(2), and two equivalent C(1) atoms. There are five inequivalent Co sites. In the first Co site, Co(1) is bonded in a single-bond geometry to one Co(3), one Co(5), one Si(2), one Si(3), and one C(2) atom. In the second Co site, Co(2) is bonded in a single-bond geometry to one Co(5), two equivalent Co(3), one Si(1), one Si(2), and one C(1) atom. In the third Co site, Co(3) is bonded to two equivalent Sm(1), one Co(1), one Co(5), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), one Si(1), and one Si(2) atom to form distorted CoSm2Co8Si2 cuboctahedra that share corners with two equivalent Co(3)Sm2Co8Si2 cuboctahedra, corners with three equivalent C(1)Sm2Co4 octahedra, corners with three equivalent C(2)Sm2Co4 octahedra,  an edgeedge with one C(1)Sm2Co4 octahedra, faces with four equivalent Co(3)Sm2Co8Si2 cuboctahedra, and  a faceface with one C(1)Sm2Co4 octahedra. The corner-sharing octahedral tilt angles range from 24-70°. In the fourth Co site, Co(4) is bonded in a single-bond geometry to one Co(5), two equivalent Co(3), one Si(2), one Si(3), and one C(1) atom. In the fifth Co site, Co(5) is bonded in a 14-coordinate geometry to two equivalent Sm(1), two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), one Si(1), one Si(3), and two equivalent Si(2) atoms. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to two equivalent Co(5), four equivalent Co(2), and four equivalent Co(3) atoms. In the second Si site, Si(2) is bonded in a 10-coordinate geometry to two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), and two equivalent Co(5) atoms. In the third Si site, Si(3) is bonded in a 10-coordinate geometry to two equivalent Co(5), four equivalent Co(1), and four equivalent Co(4) atoms. There are two inequivalent C sites. In the first C site, C(1) is bonded to two equivalent Sm(1), two equivalent Co(2), and two equivalent Co(4) atoms to form CSm2Co4 octahedra that share corners with six equivalent Co(3)Sm2Co8Si2 cuboctahedra, corners with two equivalent C(1)Sm2Co4 octahedra, corners with two equivalent C(2)Sm2Co4 octahedra, edges with two equivalent Co(3)Sm2Co8Si2 cuboctahedra, and faces with two equivalent Co(3)Sm2Co8Si2 cuboctahedra. The corner-sharing octahedral tilt angles range from 37-86°. In the second C site, C(2) is bonded to two equivalent Sm(1) and four equivalent Co(1) atoms to form CSm2Co4 octahedra that share corners with twelve equivalent Co(3)Sm2Co8Si2 cuboctahedra and corners with four equivalent C(1)Sm2Co4 octahedra. The corner-sharing octahedral tilt angles are 86°.
Sm2Co18Si4C3 crystallizes in the monoclinic C2/m space group. Sm(1) is bonded in a 3-coordinate geometry to two equivalent Co(5), four equivalent Co(3), one C(2), and two equivalent C(1) atoms. There is one shorter (3.13 Å) and one longer (3.27 Å) Sm(1)-Co(5) bond length. There are two shorter (3.05 Å) and two longer (3.14 Å) Sm(1)-Co(3) bond lengths. The Sm(1)-C(2) bond length is 2.63 Å. Both Sm(1)-C(1) bond lengths are 2.60 Å. There are five inequivalent Co sites. In the first Co site, Co(1) is bonded in a single-bond geometry to one Co(3), one Co(5), one Si(2), one Si(3), and one C(2) atom. The Co(1)-Co(3) bond length is 2.60 Å. The Co(1)-Co(5) bond length is 2.63 Å. The Co(1)-Si(2) bond length is 2.38 Å. The Co(1)-Si(3) bond length is 2.35 Å. The Co(1)-C(2) bond length is 1.83 Å. In the second Co site, Co(2) is bonded in a single-bond geometry to one Co(5), two equivalent Co(3), one Si(1), one Si(2), and one C(1) atom. The Co(2)-Co(5) bond length is 2.63 Å. There is one shorter (2.48 Å) and one longer (2.59 Å) Co(2)-Co(3) bond length. The Co(2)-Si(1) bond length is 2.36 Å. The Co(2)-Si(2) bond length is 2.37 Å. The Co(2)-C(1) bond length is 1.83 Å. In the third Co site, Co(3) is bonded to two equivalent Sm(1), one Co(1), one Co(5), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), one Si(1), and one Si(2) atom to form distorted CoSm2Co8Si2 cuboctahedra that share corners with two equivalent Co(3)Sm2Co8Si2 cuboctahedra, corners with three equivalent C(1)Sm2Co4 octahedra, corners with three equivalent C(2)Sm2Co4 octahedra,  an edgeedge with one C(1)Sm2Co4 octahedra, faces with four equivalent Co(3)Sm2Co8Si2 cuboctahedra, and  a faceface with one C(1)Sm2Co4 octahedra. The corner-sharing octahedral tilt angles range from 24-70°. The Co(3)-Co(5) bond length is 2.60 Å. There is one shorter (2.39 Å) and one longer (2.56 Å) Co(3)-Co(3) bond length. There is one shorter (2.42 Å) and one longer (2.52 Å) Co(3)-Co(4) bond length. The Co(3)-Si(1) bond length is 2.37 Å. The Co(3)-Si(2) bond length is 2.42 Å. In the fourth Co site, Co(4) is bonded in a single-bond geometry to one Co(5), two equivalent Co(3), one Si(2), one Si(3), and one C(1) atom. The Co(4)-Co(5) bond length is 2.69 Å. The Co(4)-Si(2) bond length is 2.34 Å. The Co(4)-Si(3) bond length is 2.41 Å. The Co(4)-C(1) bond length is 1.83 Å. In the fifth Co site, Co(5) is bonded in a 14-coordinate geometry to two equivalent Sm(1), two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), one Si(1), one Si(3), and two equivalent Si(2) atoms. The Co(5)-Si(1) bond length is 2.60 Å. The Co(5)-Si(3) bond length is 2.62 Å. Both Co(5)-Si(2) bond lengths are 2.58 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to two equivalent Co(5), four equivalent Co(2), and four equivalent Co(3) atoms. In the second Si site, Si(2) is bonded in a 10-coordinate geometry to two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), two equivalent Co(4), and two equivalent Co(5) atoms. In the third Si site, Si(3) is bonded in a 10-coordinate geometry to two equivalent Co(5), four equivalent Co(1), and four equivalent Co(4) atoms. There are two inequivalent C sites. In the first C site, C(1) is bonded to two equivalent Sm(1), two equivalent Co(2), and two equivalent Co(4) atoms to form CSm2Co4 octahedra that share corners with six equivalent Co(3)Sm2Co8Si2 cuboctahedra, corners with two equivalent C(1)Sm2Co4 octahedra, corners with two equivalent C(2)Sm2Co4 octahedra, edges with two equivalent Co(3)Sm2Co8Si2 cuboctahedra, and faces with two equivalent Co(3)Sm2Co8Si2 cuboctahedra. The corner-sharing octahedral tilt angles range from 37-86°. In the second C site, C(2) is bonded to two equivalent Sm(1) and four equivalent Co(1) atoms to form CSm2Co4 octahedra that share corners with twelve equivalent Co(3)Sm2Co8Si2 cuboctahedra and corners with four equivalent C(1)Sm2Co4 octahedra. The corner-sharing octahedral tilt angles are 86°.