CeSe2 crystallizes in the triclinic P1 space group. There are three inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(1), one Se(2), one Se(4), one Se(6), and two equivalent Se(3) atoms to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(3)Se7 pentagonal bipyramid, corners with two equivalent Ce(2)Se7 pentagonal bipyramids, corners with two equivalent Ce(1)Se6 pentagonal pyramids, edges with two equivalent Ce(2)Se7 pentagonal bipyramids, and edges with three equivalent Ce(3)Se7 pentagonal bipyramids. In the second Ce site, Ce(2) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), one Se(6), and two equivalent Se(5) atoms to form distorted CeSe7 pentagonal bipyramids that share corners with two equivalent Ce(2)Se7 pentagonal bipyramids, corners with two equivalent Ce(1)Se6 pentagonal pyramids, edges with four equivalent Ce(3)Se7 pentagonal bipyramids, and edges with two equivalent Ce(1)Se6 pentagonal pyramids. In the third Ce site, Ce(3) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), one Se(6), and two equivalent Se(4) atoms to form distorted CeSe7 pentagonal bipyramids that share corners with two equivalent Ce(3)Se7 pentagonal bipyramids,  a cornercorner with one Ce(1)Se6 pentagonal pyramid, edges with four equivalent Ce(2)Se7 pentagonal bipyramids, and edges with three equivalent Ce(1)Se6 pentagonal pyramids. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the third Se site, Se(3) is bonded to one Ce(2), one Ce(3), and two equivalent Ce(1) atoms to form a mixture of corner and edge-sharing SeCe4 tetrahedra. In the fourth Se site, Se(4) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form a mixture of distorted corner and edge-sharing SeCe4 trigonal pyramids. In the fifth Se site, Se(5) is bonded in a trigonal non-coplanar geometry to one Ce(3) and two equivalent Ce(2) atoms. In the sixth Se site, Se(6) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom.
CeSe2 crystallizes in the triclinic P1 space group. There are three inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(1), one Se(2), one Se(4), one Se(6), and two equivalent Se(3) atoms to form distorted CeSe6 pentagonal pyramids that share  a cornercorner with one Ce(3)Se7 pentagonal bipyramid, corners with two equivalent Ce(2)Se7 pentagonal bipyramids, corners with two equivalent Ce(1)Se6 pentagonal pyramids, edges with two equivalent Ce(2)Se7 pentagonal bipyramids, and edges with three equivalent Ce(3)Se7 pentagonal bipyramids. The Ce(1)-Se(1) bond length is 2.80 Å. The Ce(1)-Se(2) bond length is 2.79 Å. The Ce(1)-Se(4) bond length is 2.90 Å. The Ce(1)-Se(6) bond length is 2.80 Å. There is one shorter (2.96 Å) and one longer (3.00 Å) Ce(1)-Se(3) bond length. In the second Ce site, Ce(2) is bonded to one Se(1), one Se(2), one Se(3), one Se(4), one Se(6), and two equivalent Se(5) atoms to form distorted CeSe7 pentagonal bipyramids that share corners with two equivalent Ce(2)Se7 pentagonal bipyramids, corners with two equivalent Ce(1)Se6 pentagonal pyramids, edges with four equivalent Ce(3)Se7 pentagonal bipyramids, and edges with two equivalent Ce(1)Se6 pentagonal pyramids. The Ce(2)-Se(1) bond length is 2.91 Å. The Ce(2)-Se(2) bond length is 2.93 Å. The Ce(2)-Se(3) bond length is 3.00 Å. The Ce(2)-Se(4) bond length is 2.96 Å. The Ce(2)-Se(6) bond length is 2.89 Å. There is one shorter (2.91 Å) and one longer (2.93 Å) Ce(2)-Se(5) bond length. In the third Ce site, Ce(3) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), one Se(6), and two equivalent Se(4) atoms to form distorted CeSe7 pentagonal bipyramids that share corners with two equivalent Ce(3)Se7 pentagonal bipyramids,  a cornercorner with one Ce(1)Se6 pentagonal pyramid, edges with four equivalent Ce(2)Se7 pentagonal bipyramids, and edges with three equivalent Ce(1)Se6 pentagonal pyramids. The Ce(3)-Se(1) bond length is 2.90 Å. The Ce(3)-Se(2) bond length is 2.89 Å. The Ce(3)-Se(3) bond length is 2.95 Å. The Ce(3)-Se(5) bond length is 2.90 Å. The Ce(3)-Se(6) bond length is 2.87 Å. There is one shorter (2.93 Å) and one longer (2.95 Å) Ce(3)-Se(4) bond length. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom. In the third Se site, Se(3) is bonded to one Ce(2), one Ce(3), and two equivalent Ce(1) atoms to form a mixture of corner and edge-sharing SeCe4 tetrahedra. In the fourth Se site, Se(4) is bonded to one Ce(1), one Ce(2), and two equivalent Ce(3) atoms to form a mixture of distorted corner and edge-sharing SeCe4 trigonal pyramids. In the fifth Se site, Se(5) is bonded in a trigonal non-coplanar geometry to one Ce(3) and two equivalent Ce(2) atoms. In the sixth Se site, Se(6) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(3) atom.