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NiSe is Millerite structured and crystallizes in the trigonal R3m space group. The structure is three-dimensional. Ni2+ is bonded to five equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe5 trigonal bipyramids. There are three shorter (2.39 Å) and two longer (2.47 Å) Ni–Se bond lengths. Se2- is bonded in a 5-coordinate geometry to five equivalent Ni2+ atoms.
Ni3Se2 is Hazelwoodite structured and crystallizes in the trigonal R32 space group. The structure is three-dimensional. Ni+1.33+ is bonded to four equivalent Se2- atoms to form a mixture of distorted edge and corner-sharing NiSe4 tetrahedra. There are two shorter (2.37 Å) and two longer (2.39 Å) Ni–Se bond lengths. Se2- is bonded in a 6-coordinate geometry to six equivalent Ni+1.33+ atoms.
NiSe2 is pyrite-like structured and crystallizes in the cubic Pa-3 space group. The structure is three-dimensional. Ni4+ is bonded to six equivalent Se2- atoms to form corner-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 63°. All Ni–Se bond lengths are 2.46 Å. Se2- is bonded in a 3-coordinate geometry to three equivalent Ni4+ atoms.
NiSe is lead oxide structured and crystallizes in the tetragonal P4/nmm space group. The structure is two-dimensional and consists of one NiSe sheet oriented in the (0, 0, 1) direction. Ni2+ is bonded to four equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe4 tetrahedra. All Ni–Se bond lengths are 2.36 Å. Se2- is bonded in a 4-coordinate geometry to four equivalent Ni2+ atoms.
Ni3Se4 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are two inequivalent Ni+2.67+ sites. In the first Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form a mixture of edge, face, and corner-sharing NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are two shorter (2.45 Å) and four longer (2.50 Å) Ni–Se bond lengths. In the second Ni+2.67+ site, Ni+2.67+ is bonded to six Se2- atoms to form a mixture of edge, face, and corner-sharing NiSe6 octahedra. The corner-sharing octahedra tilt angles range from 49–54°. There are a spread of Ni–Se bond distances ranging from 2.40–2.50 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to four Ni+2.67+ atoms to form a mixture of distorted edge and corner-sharing SeNi4 trigonal pyramids. In the second Se2- site, Se2- is bonded in a 5-coordinate geometry to five Ni+2.67+ atoms.
NiSe is Molybdenum Carbide MAX Phase-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Ni2+ is bonded to six equivalent Se2- atoms to form a mixture of edge, corner, and face-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 50°. All Ni–Se bond lengths are 2.50 Å. Se2- is bonded in a 6-coordinate geometry to six equivalent Ni2+ atoms.
NiSe2 is Marcasite structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Ni4+ is bonded to six equivalent Se2- atoms to form a mixture of edge and corner-sharing NiSe6 octahedra. The corner-sharing octahedral tilt angles are 59°. All Ni–Se bond lengths are 2.46 Å. Se2- is bonded in a 4-coordinate geometry to three equivalent Ni4+ atoms.