CeSe2 crystallizes in the monoclinic P2_1/c space group. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), and two equivalent Se(5) atoms to form CeSe6 octahedra that share corners with two equivalent Ce(1)Se6 octahedra,  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(2)Se6 pentagonal pyramid, edges with two equivalent Ce(4)Se7 pentagonal bipyramids,  an edgeedge with one Ce(3)Se6 pentagonal pyramid, and edges with two equivalent Ce(2)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 57°. In the second Ce site, Ce(2) is bonded to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), and one Se(8) atom to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(1)Se6 octahedra,  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(3)Se6 pentagonal pyramid, edges with two equivalent Ce(1)Se6 octahedra,  an edgeedge with one Ce(4)Se7 pentagonal bipyramid, and edges with two equivalent Ce(3)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 45°. In the third Ce site, Ce(3) is bonded to one Se(2), one Se(3), one Se(6), one Se(7), and two equivalent Se(8) atoms to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(2)Se6 pentagonal pyramid,  an edgeedge with one Ce(1)Se6 octahedra,  an edgeedge with one Ce(4)Se7 pentagonal bipyramid,  an edgeedge with one Ce(3)Se6 pentagonal pyramid, and edges with two equivalent Ce(2)Se6 pentagonal pyramids. In the fourth Ce site, Ce(4) is bonded to one Se(4), one Se(5), one Se(6), two equivalent Se(1), and two equivalent Se(7) atoms to form distorted CeSe7 pentagonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra,  a cornercorner with one Ce(2)Se6 pentagonal pyramid,  a cornercorner with one Ce(3)Se6 pentagonal pyramid, edges with two equivalent Ce(1)Se6 octahedra, edges with two equivalent Ce(4)Se7 pentagonal bipyramids,  an edgeedge with one Ce(2)Se6 pentagonal pyramid, and  an edgeedge with one Ce(3)Se6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 29°. There are eight inequivalent Se sites. In the first Se site, Se(5) is bonded in a see-saw-like geometry to one Ce(2), one Ce(4), and two equivalent Ce(1) atoms. In the second Se site, Se(6) is bonded in a distorted trigonal non-coplanar geometry to one Ce(2), one Ce(3), and one Ce(4) atom. In the third Se site, Se(7) is bonded in a T-shaped geometry to one Ce(3) and two equivalent Ce(4) atoms. In the fourth Se site, Se(8) is bonded in a distorted trigonal non-coplanar geometry to one Ce(2) and two equivalent Ce(3) atoms. In the fifth Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one Ce(1) and two equivalent Ce(4) atoms. In the sixth Se site, Se(2) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the seventh Se site, Se(3) is bonded in a distorted trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the eighth Se site, Se(4) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(4) atom.
CeSe2 crystallizes in the monoclinic P2_1/c space group. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), and two equivalent Se(5) atoms to form CeSe6 octahedra that share corners with two equivalent Ce(1)Se6 octahedra,  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(2)Se6 pentagonal pyramid, edges with two equivalent Ce(4)Se7 pentagonal bipyramids,  an edgeedge with one Ce(3)Se6 pentagonal pyramid, and edges with two equivalent Ce(2)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 57°. The Ce(1)-Se(1) bond length is 2.82 Å. The Ce(1)-Se(2) bond length is 2.83 Å. The Ce(1)-Se(3) bond length is 2.83 Å. The Ce(1)-Se(4) bond length is 2.88 Å. There is one shorter (2.88 Å) and one longer (2.91 Å) Ce(1)-Se(5) bond length. In the second Ce site, Ce(2) is bonded to one Se(2), one Se(3), one Se(4), one Se(5), one Se(6), and one Se(8) atom to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(1)Se6 octahedra,  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(3)Se6 pentagonal pyramid, edges with two equivalent Ce(1)Se6 octahedra,  an edgeedge with one Ce(4)Se7 pentagonal bipyramid, and edges with two equivalent Ce(3)Se6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 45°. The Ce(2)-Se(2) bond length is 2.88 Å. The Ce(2)-Se(3) bond length is 2.87 Å. The Ce(2)-Se(4) bond length is 2.89 Å. The Ce(2)-Se(5) bond length is 2.93 Å. The Ce(2)-Se(6) bond length is 2.79 Å. The Ce(2)-Se(8) bond length is 2.91 Å. In the third Ce site, Ce(3) is bonded to one Se(2), one Se(3), one Se(6), one Se(7), and two equivalent Se(8) atoms to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(4)Se7 pentagonal bipyramid,  a cornercorner with one Ce(2)Se6 pentagonal pyramid,  an edgeedge with one Ce(1)Se6 octahedra,  an edgeedge with one Ce(4)Se7 pentagonal bipyramid,  an edgeedge with one Ce(3)Se6 pentagonal pyramid, and edges with two equivalent Ce(2)Se6 pentagonal pyramids. The Ce(3)-Se(2) bond length is 2.92 Å. The Ce(3)-Se(3) bond length is 2.91 Å. The Ce(3)-Se(6) bond length is 2.88 Å. The Ce(3)-Se(7) bond length is 2.86 Å. There is one shorter (2.80 Å) and one longer (2.87 Å) Ce(3)-Se(8) bond length. In the fourth Ce site, Ce(4) is bonded to one Se(4), one Se(5), one Se(6), two equivalent Se(1), and two equivalent Se(7) atoms to form distorted CeSe7 pentagonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra,  a cornercorner with one Ce(2)Se6 pentagonal pyramid,  a cornercorner with one Ce(3)Se6 pentagonal pyramid, edges with two equivalent Ce(1)Se6 octahedra, edges with two equivalent Ce(4)Se7 pentagonal bipyramids,  an edgeedge with one Ce(2)Se6 pentagonal pyramid, and  an edgeedge with one Ce(3)Se6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 29°. The Ce(4)-Se(4) bond length is 3.01 Å. The Ce(4)-Se(5) bond length is 3.06 Å. The Ce(4)-Se(6) bond length is 2.86 Å. There is one shorter (2.85 Å) and one longer (2.98 Å) Ce(4)-Se(1) bond length. There is one shorter (2.91 Å) and one longer (2.94 Å) Ce(4)-Se(7) bond length. There are eight inequivalent Se sites. In the first Se site, Se(5) is bonded in a see-saw-like geometry to one Ce(2), one Ce(4), and two equivalent Ce(1) atoms. In the second Se site, Se(6) is bonded in a distorted trigonal non-coplanar geometry to one Ce(2), one Ce(3), and one Ce(4) atom. In the third Se site, Se(7) is bonded in a T-shaped geometry to one Ce(3) and two equivalent Ce(4) atoms. In the fourth Se site, Se(8) is bonded in a distorted trigonal non-coplanar geometry to one Ce(2) and two equivalent Ce(3) atoms. In the fifth Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one Ce(1) and two equivalent Ce(4) atoms. In the sixth Se site, Se(2) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the seventh Se site, Se(3) is bonded in a distorted trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the eighth Se site, Se(4) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(4) atom.