Tm2CoSi2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 10-coordinate geometry to four equivalent Co(1), two equivalent Si(2), and five equivalent Si(1) atoms. In the second Tm site, Tm(2) is bonded in a 6-coordinate geometry to two equivalent Co(1), one Si(1), and five equivalent Si(2) atoms. Co(1) is bonded in a 10-coordinate geometry to two equivalent Tm(2), four equivalent Tm(1), one Si(2), and three equivalent Si(1) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to one Tm(2), five equivalent Tm(1), three equivalent Co(1), and one Si(1) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to two equivalent Tm(1), five equivalent Tm(2), one Co(1), and one Si(2) atom.
Tm2CoSi2 crystallizes in the monoclinic C2/m space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 10-coordinate geometry to four equivalent Co(1), two equivalent Si(2), and five equivalent Si(1) atoms. There are a spread of Tm(1)-Co(1) bond distances ranging from 2.89-3.02 Å. Both Tm(1)-Si(2) bond lengths are 2.85 Å. There are a spread of Tm(1)-Si(1) bond distances ranging from 2.91-3.16 Å. In the second Tm site, Tm(2) is bonded in a 6-coordinate geometry to two equivalent Co(1), one Si(1), and five equivalent Si(2) atoms. Both Tm(2)-Co(1) bond lengths are 3.21 Å. The Tm(2)-Si(1) bond length is 2.84 Å. There are a spread of Tm(2)-Si(2) bond distances ranging from 2.91-3.03 Å. Co(1) is bonded in a 10-coordinate geometry to two equivalent Tm(2), four equivalent Tm(1), one Si(2), and three equivalent Si(1) atoms. The Co(1)-Si(2) bond length is 2.44 Å. There are two shorter (2.32 Å) and one longer (2.38 Å) Co(1)-Si(1) bond length. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded in a 10-coordinate geometry to one Tm(2), five equivalent Tm(1), three equivalent Co(1), and one Si(1) atom. The Si(1)-Si(1) bond length is 2.67 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to two equivalent Tm(1), five equivalent Tm(2), one Co(1), and one Si(2) atom. The Si(2)-Si(2) bond length is 2.51 Å.