IrSe2 is Marcasite-like structured and crystallizes in the orthorhombic Pnma space group. There are two inequivalent Ir sites. In the first Ir site, Ir(1) is bonded to one Se(1), two equivalent Se(4), and three equivalent Se(2) atoms to form IrSe6 octahedra that share corners with four equivalent Ir(1)Se6 octahedra, corners with four equivalent Ir(2)Se6 octahedra,  a cornercorner with one Se(2)Ir3Se tetrahedra, and edges with two equivalent Ir(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-62°. In the second Ir site, Ir(2) is bonded to one Se(4), two equivalent Se(1), and three equivalent Se(3) atoms to form IrSe6 octahedra that share corners with four equivalent Ir(1)Se6 octahedra, corners with two equivalent Se(2)Ir3Se tetrahedra, and edges with four equivalent Ir(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-62°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one Ir(1), two equivalent Ir(2), and one Se(2) atom. In the second Se site, Se(2) is bonded to three equivalent Ir(1) and one Se(1) atom to form distorted SeIr3Se tetrahedra that share  a cornercorner with one Ir(1)Se6 octahedra, corners with two equivalent Ir(2)Se6 octahedra, and corners with six equivalent Se(2)Ir3Se tetrahedra. The corner-sharing octahedral tilt angles range from 77-82°. In the third Se site, Se(3) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Ir(2) atoms. In the fourth Se site, Se(4) is bonded in a trigonal non-coplanar geometry to one Ir(2) and two equivalent Ir(1) atoms.
IrSe2 is Marcasite-like structured and crystallizes in the orthorhombic Pnma space group. There are two inequivalent Ir sites. In the first Ir site, Ir(1) is bonded to one Se(1), two equivalent Se(4), and three equivalent Se(2) atoms to form IrSe6 octahedra that share corners with four equivalent Ir(1)Se6 octahedra, corners with four equivalent Ir(2)Se6 octahedra,  a cornercorner with one Se(2)Ir3Se tetrahedra, and edges with two equivalent Ir(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-62°. The Ir(1)-Se(1) bond length is 2.51 Å. Both Ir(1)-Se(4) bond lengths are 2.52 Å. There is one shorter (2.43 Å) and two longer (2.47 Å) Ir(1)-Se(2) bond lengths. In the second Ir site, Ir(2) is bonded to one Se(4), two equivalent Se(1), and three equivalent Se(3) atoms to form IrSe6 octahedra that share corners with four equivalent Ir(1)Se6 octahedra, corners with two equivalent Se(2)Ir3Se tetrahedra, and edges with four equivalent Ir(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-62°. The Ir(2)-Se(4) bond length is 2.48 Å. Both Ir(2)-Se(1) bond lengths are 2.45 Å. There is one shorter (2.52 Å) and two longer (2.54 Å) Ir(2)-Se(3) bond lengths. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one Ir(1), two equivalent Ir(2), and one Se(2) atom. The Se(1)-Se(2) bond length is 2.60 Å. In the second Se site, Se(2) is bonded to three equivalent Ir(1) and one Se(1) atom to form distorted SeIr3Se tetrahedra that share  a cornercorner with one Ir(1)Se6 octahedra, corners with two equivalent Ir(2)Se6 octahedra, and corners with six equivalent Se(2)Ir3Se tetrahedra. The corner-sharing octahedral tilt angles range from 77-82°. In the third Se site, Se(3) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Ir(2) atoms. In the fourth Se site, Se(4) is bonded in a trigonal non-coplanar geometry to one Ir(2) and two equivalent Ir(1) atoms.