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.