Fe(CoSe2)2 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form FeSe6 octahedra that share corners with twelve equivalent Co(1)Se6 octahedra, edges with two equivalent Fe(1)Se6 octahedra, and faces with two equivalent Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. Co(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form distorted CoSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with six equivalent Co(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to two equivalent Fe(1) and three equivalent Co(1) atoms. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Fe(1) and three equivalent Co(1) atoms.
Fe(CoSe2)2 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form FeSe6 octahedra that share corners with twelve equivalent Co(1)Se6 octahedra, edges with two equivalent Fe(1)Se6 octahedra, and faces with two equivalent Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. Both Fe(1)-Se(1) bond lengths are 2.45 Å. All Fe(1)-Se(2) bond lengths are 2.50 Å. Co(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form distorted CoSe6 octahedra that share corners with six equivalent Fe(1)Se6 octahedra, edges with six equivalent Co(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 51-52°. There are two shorter (2.35 Å) and one longer (2.38 Å) Co(1)-Se(1) bond length. There is one shorter (2.44 Å) and two longer (2.48 Å) Co(1)-Se(2) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to two equivalent Fe(1) and three equivalent Co(1) atoms. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Fe(1) and three equivalent Co(1) atoms.