CeSe2 crystallizes in the triclinic P1 space group. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(2), one Se(3), one Se(5), one Se(6), one Se(7), and one Se(8) atom to form distorted CeSe6 octahedra that share  a cornercorner with one Ce(2)Se5 trigonal bipyramid,  a cornercorner with one Ce(4)Se5 trigonal bipyramid,  an edgeedge with one Ce(2)Se5 trigonal bipyramid, edges with two equivalent Ce(4)Se5 trigonal bipyramids, and  an edgeedge with one Ce(3)Se4 trigonal pyramid. In the second Ce site, Ce(2) is bonded to one Se(1), one Se(2), one Se(4), one Se(6), and one Se(7) atom to form CeSe5 trigonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra, corners with two equivalent Ce(4)Se5 trigonal bipyramids, corners with two equivalent Ce(3)Se4 trigonal pyramids, and  an edgeedge with one Ce(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 53°. In the third Ce site, Ce(3) is bonded to one Se(1), one Se(3), one Se(4), and one Se(8) atom to form CeSe4 trigonal pyramids that share corners with two equivalent Ce(2)Se5 trigonal bipyramids, corners with two equivalent Ce(4)Se5 trigonal bipyramids, and  an edgeedge with one Ce(1)Se6 octahedra. In the fourth Ce site, Ce(4) is bonded to one Se(3), one Se(5), one Se(6), one Se(7), and one Se(8) atom to form CeSe5 trigonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra, corners with two equivalent Ce(2)Se5 trigonal bipyramids, corners with two equivalent Ce(3)Se4 trigonal pyramids, and edges with two equivalent Ce(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 36°. There are eight inequivalent Se sites. In the first Se site, Se(1) is bonded in a bent 120 degrees geometry to one Ce(2) and one Ce(3) atom. In the second Se site, Se(2) is bonded in a bent 120 degrees geometry to one Ce(1) and one Ce(2) atom. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ce(1), one Ce(3), and one Ce(4) atom. In the fourth Se site, Se(4) is bonded in a bent 120 degrees geometry to one Ce(2) and one Ce(3) atom. In the fifth Se site, Se(5) is bonded in a bent 150 degrees geometry to one Ce(1) and one Ce(4) atom. In the sixth Se site, Se(6) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(4) atom. In the seventh Se site, Se(7) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(4) atom. In the eighth Se site, Se(8) is bonded in a T-shaped geometry to one Ce(1), one Ce(3), and one Ce(4) atom.
CeSe2 crystallizes in the triclinic P1 space group. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded to one Se(2), one Se(3), one Se(5), one Se(6), one Se(7), and one Se(8) atom to form distorted CeSe6 octahedra that share  a cornercorner with one Ce(2)Se5 trigonal bipyramid,  a cornercorner with one Ce(4)Se5 trigonal bipyramid,  an edgeedge with one Ce(2)Se5 trigonal bipyramid, edges with two equivalent Ce(4)Se5 trigonal bipyramids, and  an edgeedge with one Ce(3)Se4 trigonal pyramid. The Ce(1)-Se(2) bond length is 2.76 Å. The Ce(1)-Se(3) bond length is 2.94 Å. The Ce(1)-Se(5) bond length is 2.82 Å. The Ce(1)-Se(6) bond length is 2.88 Å. The Ce(1)-Se(7) bond length is 2.92 Å. The Ce(1)-Se(8) bond length is 2.96 Å. In the second Ce site, Ce(2) is bonded to one Se(1), one Se(2), one Se(4), one Se(6), and one Se(7) atom to form CeSe5 trigonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra, corners with two equivalent Ce(4)Se5 trigonal bipyramids, corners with two equivalent Ce(3)Se4 trigonal pyramids, and  an edgeedge with one Ce(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 53°. The Ce(2)-Se(1) bond length is 2.81 Å. The Ce(2)-Se(2) bond length is 2.72 Å. The Ce(2)-Se(4) bond length is 2.80 Å. The Ce(2)-Se(6) bond length is 2.88 Å. The Ce(2)-Se(7) bond length is 2.83 Å. In the third Ce site, Ce(3) is bonded to one Se(1), one Se(3), one Se(4), and one Se(8) atom to form CeSe4 trigonal pyramids that share corners with two equivalent Ce(2)Se5 trigonal bipyramids, corners with two equivalent Ce(4)Se5 trigonal bipyramids, and  an edgeedge with one Ce(1)Se6 octahedra. The Ce(3)-Se(1) bond length is 2.71 Å. The Ce(3)-Se(3) bond length is 2.78 Å. The Ce(3)-Se(4) bond length is 2.69 Å. The Ce(3)-Se(8) bond length is 2.79 Å. In the fourth Ce site, Ce(4) is bonded to one Se(3), one Se(5), one Se(6), one Se(7), and one Se(8) atom to form CeSe5 trigonal bipyramids that share  a cornercorner with one Ce(1)Se6 octahedra, corners with two equivalent Ce(2)Se5 trigonal bipyramids, corners with two equivalent Ce(3)Se4 trigonal pyramids, and edges with two equivalent Ce(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 36°. The Ce(4)-Se(3) bond length is 2.89 Å. The Ce(4)-Se(5) bond length is 2.69 Å. The Ce(4)-Se(6) bond length is 2.80 Å. The Ce(4)-Se(7) bond length is 2.84 Å. The Ce(4)-Se(8) bond length is 2.88 Å. There are eight inequivalent Se sites. In the first Se site, Se(1) is bonded in a bent 120 degrees geometry to one Ce(2) and one Ce(3) atom. In the second Se site, Se(2) is bonded in a bent 120 degrees geometry to one Ce(1) and one Ce(2) atom. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ce(1), one Ce(3), and one Ce(4) atom. In the fourth Se site, Se(4) is bonded in a bent 120 degrees geometry to one Ce(2) and one Ce(3) atom. In the fifth Se site, Se(5) is bonded in a bent 150 degrees geometry to one Ce(1) and one Ce(4) atom. In the sixth Se site, Se(6) is bonded in a T-shaped geometry to one Ce(1), one Ce(2), and one Ce(4) atom. In the seventh Se site, Se(7) is bonded in a trigonal non-coplanar geometry to one Ce(1), one Ce(2), and one Ce(4) atom. In the eighth Se site, Se(8) is bonded in a T-shaped geometry to one Ce(1), one Ce(3), and one Ce(4) atom.