Np3Se5 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Np sites. In the first Np site, Np(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. In the second Np site, Np(2) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing NpSe7 pentagonal bipyramids. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Np(1) and two equivalent Np(2) atoms to form distorted SeNp4 trigonal pyramids that share corners with two equivalent Se(4)Np5 square pyramids, corners with four equivalent Se(2)Np5 square pyramids, corners with two equivalent Se(3)Np5 trigonal bipyramids, corners with six equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(2)Np5 square pyramid, edges with two equivalent Se(4)Np5 square pyramids, edges with two equivalent Se(3)Np5 trigonal bipyramids, and  an edgeedge with one Se(1)Np4 trigonal pyramid. In the second Se site, Se(2) is bonded to one Np(2) and four equivalent Np(1) atoms to form SeNp5 square pyramids that share corners with four equivalent Se(4)Np5 square pyramids, corners with five equivalent Se(3)Np5 trigonal bipyramids, corners with eight equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(4)Np5 square pyramid, edges with two equivalent Se(2)Np5 square pyramids, edges with two equivalent Se(3)Np5 trigonal bipyramids, edges with two equivalent Se(1)Np4 trigonal pyramids, and  a faceface with one Se(4)Np5 square pyramid. In the third Se site, Se(3) is bonded to one Np(2) and four equivalent Np(1) atoms to form distorted SeNp5 trigonal bipyramids that share  a cornercorner with one Se(4)Np5 square pyramid, corners with five equivalent Se(2)Np5 square pyramids, corners with four equivalent Se(3)Np5 trigonal bipyramids, corners with four equivalent Se(1)Np4 trigonal pyramids, edges with two equivalent Se(2)Np5 square pyramids, edges with four equivalent Se(4)Np5 square pyramids, and edges with four equivalent Se(1)Np4 trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Np(2) and four equivalent Np(1) atoms to form SeNp5 square pyramids that share corners with four equivalent Se(2)Np5 square pyramids, corners with four equivalent Se(4)Np5 square pyramids,  a cornercorner with one Se(3)Np5 trigonal bipyramid, corners with four equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(2)Np5 square pyramid, edges with four equivalent Se(3)Np5 trigonal bipyramids, edges with four equivalent Se(1)Np4 trigonal pyramids, and  a faceface with one Se(2)Np5 square pyramid.
Np3Se5 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Np sites. In the first Np site, Np(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(4) atoms. There is one shorter (2.96 Å) and one longer (3.02 Å) Np(1)-Se(1) bond length. There is one shorter (3.07 Å) and one longer (3.09 Å) Np(1)-Se(2) bond length. There is one shorter (3.07 Å) and one longer (3.21 Å) Np(1)-Se(3) bond length. There is one shorter (3.00 Å) and one longer (3.09 Å) Np(1)-Se(4) bond length. In the second Np site, Np(2) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing NpSe7 pentagonal bipyramids. The Np(2)-Se(2) bond length is 3.05 Å. The Np(2)-Se(3) bond length is 2.91 Å. The Np(2)-Se(4) bond length is 2.94 Å. There are two shorter (2.81 Å) and two longer (2.85 Å) Np(2)-Se(1) bond lengths. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Np(1) and two equivalent Np(2) atoms to form distorted SeNp4 trigonal pyramids that share corners with two equivalent Se(4)Np5 square pyramids, corners with four equivalent Se(2)Np5 square pyramids, corners with two equivalent Se(3)Np5 trigonal bipyramids, corners with six equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(2)Np5 square pyramid, edges with two equivalent Se(4)Np5 square pyramids, edges with two equivalent Se(3)Np5 trigonal bipyramids, and  an edgeedge with one Se(1)Np4 trigonal pyramid. In the second Se site, Se(2) is bonded to one Np(2) and four equivalent Np(1) atoms to form SeNp5 square pyramids that share corners with four equivalent Se(4)Np5 square pyramids, corners with five equivalent Se(3)Np5 trigonal bipyramids, corners with eight equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(4)Np5 square pyramid, edges with two equivalent Se(2)Np5 square pyramids, edges with two equivalent Se(3)Np5 trigonal bipyramids, edges with two equivalent Se(1)Np4 trigonal pyramids, and  a faceface with one Se(4)Np5 square pyramid. In the third Se site, Se(3) is bonded to one Np(2) and four equivalent Np(1) atoms to form distorted SeNp5 trigonal bipyramids that share  a cornercorner with one Se(4)Np5 square pyramid, corners with five equivalent Se(2)Np5 square pyramids, corners with four equivalent Se(3)Np5 trigonal bipyramids, corners with four equivalent Se(1)Np4 trigonal pyramids, edges with two equivalent Se(2)Np5 square pyramids, edges with four equivalent Se(4)Np5 square pyramids, and edges with four equivalent Se(1)Np4 trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Np(2) and four equivalent Np(1) atoms to form SeNp5 square pyramids that share corners with four equivalent Se(2)Np5 square pyramids, corners with four equivalent Se(4)Np5 square pyramids,  a cornercorner with one Se(3)Np5 trigonal bipyramid, corners with four equivalent Se(1)Np4 trigonal pyramids,  an edgeedge with one Se(2)Np5 square pyramid, edges with four equivalent Se(3)Np5 trigonal bipyramids, edges with four equivalent Se(1)Np4 trigonal pyramids, and  a faceface with one Se(2)Np5 square pyramid.