Ti2NiSe4 crystallizes in the monoclinic C2/m space group. Ti(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form TiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Ti(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. Ni(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NiSe6 octahedra that share corners with twelve equivalent Ti(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Ti(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a rectangular see-saw-like geometry to three equivalent Ti(1) and one Ni(1) atom. In the second Se site, Se(1) is bonded to three equivalent Ti(1) and two equivalent Ni(1) atoms to form a mixture of distorted corner and edge-sharing SeTi3Ni2 square pyramids.
Ti2NiSe4 crystallizes in the monoclinic C2/m space group. Ti(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form TiSe6 octahedra that share corners with six equivalent Ni(1)Se6 octahedra, edges with six equivalent Ti(1)Se6 octahedra, and  a faceface with one Ni(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. There is one shorter (2.59 Å) and two longer (2.62 Å) Ti(1)-Se(1) bond lengths. There are two shorter (2.56 Å) and one longer (2.57 Å) Ti(1)-Se(2) bond length. Ni(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form NiSe6 octahedra that share corners with twelve equivalent Ti(1)Se6 octahedra, edges with two equivalent Ni(1)Se6 octahedra, and faces with two equivalent Ti(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-49°. Both Ni(1)-Se(2) bond lengths are 2.50 Å. All Ni(1)-Se(1) bond lengths are 2.51 Å. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded in a rectangular see-saw-like geometry to three equivalent Ti(1) and one Ni(1) atom. In the second Se site, Se(1) is bonded to three equivalent Ti(1) and two equivalent Ni(1) atoms to form a mixture of distorted corner and edge-sharing SeTi3Ni2 square pyramids.