Cs(NpSe3)2 crystallizes in the orthorhombic Immm space group. Cs(1) is bonded to eight equivalent Se(1) atoms to form face-sharing CsSe8 hexagonal bipyramids. Np(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded to four equivalent Np(1) atoms to form a mixture of distorted corner and edge-sharing SeNp4 trigonal pyramids. In the second Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Np(1), and two equivalent Se(1) atoms.
Cs(NpSe3)2 crystallizes in the orthorhombic Immm space group. Cs(1) is bonded to eight equivalent Se(1) atoms to form face-sharing CsSe8 hexagonal bipyramids. All Cs(1)-Se(1) bond lengths are 3.76 Å. Np(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All Np(1)-Se(1) bond lengths are 2.96 Å. There are two shorter (2.89 Å) and two longer (2.93 Å) Np(1)-Se(2) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(2) is bonded to four equivalent Np(1) atoms to form a mixture of distorted corner and edge-sharing SeNp4 trigonal pyramids. In the second Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Np(1), and two equivalent Se(1) atoms. There is one shorter (2.65 Å) and one longer (2.80 Å) Se(1)-Se(1) bond length.