Co(SiSe2)2 crystallizes in the monoclinic C2/m space group. Co(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form CoSe6 octahedra that share corners with twelve equivalent Si(1)Se6 octahedra, edges with two equivalent Co(1)Se6 octahedra, and faces with two equivalent Si(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 44-53°. Si(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form SiSe6 octahedra that share corners with six equivalent Co(1)Se6 octahedra, edges with six equivalent Si(1)Se6 octahedra, and  a faceface with one Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 44-53°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Co(1) and three equivalent Si(1) atoms. In the second Se site, Se(1) is bonded to two equivalent Co(1) and three equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing SeCo2Si3 square pyramids.
Co(SiSe2)2 crystallizes in the monoclinic C2/m space group. Co(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form CoSe6 octahedra that share corners with twelve equivalent Si(1)Se6 octahedra, edges with two equivalent Co(1)Se6 octahedra, and faces with two equivalent Si(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 44-53°. Both Co(1)-Se(2) bond lengths are 2.46 Å. All Co(1)-Se(1) bond lengths are 2.45 Å. Si(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form SiSe6 octahedra that share corners with six equivalent Co(1)Se6 octahedra, edges with six equivalent Si(1)Se6 octahedra, and  a faceface with one Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 44-53°. All Si(1)-Se(1) bond lengths are 2.72 Å. There are two shorter (2.52 Å) and one longer (2.54 Å) Si(1)-Se(2) bond length. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Co(1) and three equivalent Si(1) atoms. In the second Se site, Se(1) is bonded to two equivalent Co(1) and three equivalent Si(1) atoms to form a mixture of distorted edge and corner-sharing SeCo2Si3 square pyramids.