Co2NiSe4 crystallizes in the monoclinic C2/m space group. Co(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form CoSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Co(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-52°. Ni(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form NiSe6 octahedra that share corners with twelve equivalent Co(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-52°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to three equivalent Co(1) and two equivalent Ni(1) atoms. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to three equivalent Co(1) and one Ni(1) atom.
Co2NiSe4 crystallizes in the monoclinic C2/m space group. Co(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form CoSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Co(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-52°. There are two shorter (2.33 Å) and one longer (2.37 Å) Co(1)-Se(1) bond length. There is one shorter (2.42 Å) and two longer (2.47 Å) Co(1)-Se(2) bond lengths. Ni(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form NiSe6 octahedra that share corners with twelve equivalent Co(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Co(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-52°. Both Ni(1)-Se(1) bond lengths are 2.47 Å. All Ni(1)-Se(2) bond lengths are 2.49 Å. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a 5-coordinate geometry to three equivalent Co(1) and two equivalent Ni(1) atoms. In the second Se site, Se(1) is bonded in a rectangular see-saw-like geometry to three equivalent Co(1) and one Ni(1) atom.