CoNi2Se4 crystallizes in the monoclinic Cm space group. Co(1) is bonded to one Se(1), one Se(3), two equivalent Se(2), and two equivalent Se(4) atoms to form distorted CoSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Co(1)Se6 octahedra, edges with four equivalent Ni(2)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 49-54°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form NiSe6 octahedra that share corners with six equivalent Co(1)Se6 octahedra, corners with six equivalent Ni(2)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra,  a faceface with one Co(1)Se6 octahedra, and  a faceface with one Ni(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 49-54°. In the second Ni site, Ni(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form distorted NiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Ni(2)Se6 octahedra, edges with four equivalent Co(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to one Co(1), one Ni(1), and two equivalent Ni(2) atoms to form SeCoNi3 trigonal pyramids that share  a cornercorner with one Se(2)Co2Ni2 trigonal pyramid, corners with two equivalent Se(1)CoNi3 trigonal pyramids, and edges with two equivalent Se(2)Co2Ni2 trigonal pyramids. In the second Se site, Se(2) is bonded to two equivalent Co(1), one Ni(1), and one Ni(2) atom to form distorted SeCo2Ni2 trigonal pyramids that share  a cornercorner with one Se(1)CoNi3 trigonal pyramid, corners with two equivalent Se(2)Co2Ni2 trigonal pyramids, and edges with two equivalent Se(1)CoNi3 trigonal pyramids. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to one Co(1), two equivalent Ni(1), and two equivalent Ni(2) atoms. In the fourth Se site, Se(4) is bonded in a 5-coordinate geometry to two equivalent Co(1), one Ni(2), and two equivalent Ni(1) atoms.
CoNi2Se4 crystallizes in the monoclinic Cm space group. Co(1) is bonded to one Se(1), one Se(3), two equivalent Se(2), and two equivalent Se(4) atoms to form distorted CoSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Co(1)Se6 octahedra, edges with four equivalent Ni(2)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 49-54°. The Co(1)-Se(1) bond length is 2.37 Å. The Co(1)-Se(3) bond length is 2.44 Å. Both Co(1)-Se(2) bond lengths are 2.36 Å. Both Co(1)-Se(4) bond lengths are 2.48 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form NiSe6 octahedra that share corners with six equivalent Co(1)Se6 octahedra, corners with six equivalent Ni(2)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra,  a faceface with one Co(1)Se6 octahedra, and  a faceface with one Ni(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 49-54°. The Ni(1)-Se(1) bond length is 2.41 Å. The Ni(1)-Se(2) bond length is 2.49 Å. Both Ni(1)-Se(3) bond lengths are 2.48 Å. Both Ni(1)-Se(4) bond lengths are 2.51 Å. In the second Ni site, Ni(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form distorted NiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with two equivalent Ni(2)Se6 octahedra, edges with four equivalent Co(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. The Ni(2)-Se(2) bond length is 2.44 Å. The Ni(2)-Se(4) bond length is 2.47 Å. Both Ni(2)-Se(1) bond lengths are 2.38 Å. Both Ni(2)-Se(3) bond lengths are 2.49 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to one Co(1), one Ni(1), and two equivalent Ni(2) atoms to form SeCoNi3 trigonal pyramids that share  a cornercorner with one Se(2)Co2Ni2 trigonal pyramid, corners with two equivalent Se(1)CoNi3 trigonal pyramids, and edges with two equivalent Se(2)Co2Ni2 trigonal pyramids. In the second Se site, Se(2) is bonded to two equivalent Co(1), one Ni(1), and one Ni(2) atom to form distorted SeCo2Ni2 trigonal pyramids that share  a cornercorner with one Se(1)CoNi3 trigonal pyramid, corners with two equivalent Se(2)Co2Ni2 trigonal pyramids, and edges with two equivalent Se(1)CoNi3 trigonal pyramids. In the third Se site, Se(3) is bonded in a 5-coordinate geometry to one Co(1), two equivalent Ni(1), and two equivalent Ni(2) atoms. In the fourth Se site, Se(4) is bonded in a 5-coordinate geometry to two equivalent Co(1), one Ni(2), and two equivalent Ni(1) atoms.