UNd2Se5 crystallizes in the orthorhombic Pnma space group. U(1) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing USe7 pentagonal bipyramids. Nd(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 are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent U(1) and two equivalent Nd(1) atoms to form distorted SeNd2U2 trigonal pyramids that share corners with two equivalent Se(4)Nd4U square pyramids, corners with four equivalent Se(3)Nd4U square pyramids, corners with two equivalent Se(2)Nd4U trigonal bipyramids, corners with six equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(3)Nd4U square pyramid, edges with two equivalent Se(4)Nd4U square pyramids, edges with two equivalent Se(2)Nd4U trigonal bipyramids, and  an edgeedge with one Se(1)Nd2U2 trigonal pyramid. In the second Se site, Se(2) is bonded to one U(1) and four equivalent Nd(1) atoms to form distorted SeNd4U trigonal bipyramids that share  a cornercorner with one Se(4)Nd4U square pyramid, corners with five equivalent Se(3)Nd4U square pyramids, corners with four equivalent Se(2)Nd4U trigonal bipyramids, corners with four equivalent Se(1)Nd2U2 trigonal pyramids, edges with two equivalent Se(3)Nd4U square pyramids, edges with four equivalent Se(4)Nd4U square pyramids, and edges with four equivalent Se(1)Nd2U2 trigonal pyramids. In the third Se site, Se(3) is bonded to one U(1) and four equivalent Nd(1) atoms to form SeNd4U square pyramids that share corners with four equivalent Se(4)Nd4U square pyramids, corners with five equivalent Se(2)Nd4U trigonal bipyramids, corners with eight equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(4)Nd4U square pyramid, edges with two equivalent Se(3)Nd4U square pyramids, edges with two equivalent Se(2)Nd4U trigonal bipyramids, edges with two equivalent Se(1)Nd2U2 trigonal pyramids, and  a faceface with one Se(4)Nd4U square pyramid. In the fourth Se site, Se(4) is bonded to one U(1) and four equivalent Nd(1) atoms to form distorted SeNd4U square pyramids that share corners with four equivalent Se(3)Nd4U square pyramids, corners with four equivalent Se(4)Nd4U square pyramids,  a cornercorner with one Se(2)Nd4U trigonal bipyramid, corners with four equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(3)Nd4U square pyramid, edges with four equivalent Se(2)Nd4U trigonal bipyramids, edges with four equivalent Se(1)Nd2U2 trigonal pyramids, and  a faceface with one Se(3)Nd4U square pyramid.
UNd2Se5 crystallizes in the orthorhombic Pnma space group. U(1) is bonded to one Se(2), one Se(3), one Se(4), and four equivalent Se(1) atoms to form distorted edge-sharing USe7 pentagonal bipyramids. The U(1)-Se(2) bond length is 2.91 Å. The U(1)-Se(3) bond length is 3.03 Å. The U(1)-Se(4) bond length is 2.95 Å. There are two shorter (2.79 Å) and two longer (2.89 Å) U(1)-Se(1) bond lengths. Nd(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 (3.03 Å) and one longer (3.04 Å) Nd(1)-Se(1) bond length. There is one shorter (3.11 Å) and one longer (3.23 Å) Nd(1)-Se(2) bond length. There is one shorter (3.06 Å) and one longer (3.11 Å) Nd(1)-Se(3) bond length. There is one shorter (3.05 Å) and one longer (3.09 Å) Nd(1)-Se(4) bond length. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent U(1) and two equivalent Nd(1) atoms to form distorted SeNd2U2 trigonal pyramids that share corners with two equivalent Se(4)Nd4U square pyramids, corners with four equivalent Se(3)Nd4U square pyramids, corners with two equivalent Se(2)Nd4U trigonal bipyramids, corners with six equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(3)Nd4U square pyramid, edges with two equivalent Se(4)Nd4U square pyramids, edges with two equivalent Se(2)Nd4U trigonal bipyramids, and  an edgeedge with one Se(1)Nd2U2 trigonal pyramid. In the second Se site, Se(2) is bonded to one U(1) and four equivalent Nd(1) atoms to form distorted SeNd4U trigonal bipyramids that share  a cornercorner with one Se(4)Nd4U square pyramid, corners with five equivalent Se(3)Nd4U square pyramids, corners with four equivalent Se(2)Nd4U trigonal bipyramids, corners with four equivalent Se(1)Nd2U2 trigonal pyramids, edges with two equivalent Se(3)Nd4U square pyramids, edges with four equivalent Se(4)Nd4U square pyramids, and edges with four equivalent Se(1)Nd2U2 trigonal pyramids. In the third Se site, Se(3) is bonded to one U(1) and four equivalent Nd(1) atoms to form SeNd4U square pyramids that share corners with four equivalent Se(4)Nd4U square pyramids, corners with five equivalent Se(2)Nd4U trigonal bipyramids, corners with eight equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(4)Nd4U square pyramid, edges with two equivalent Se(3)Nd4U square pyramids, edges with two equivalent Se(2)Nd4U trigonal bipyramids, edges with two equivalent Se(1)Nd2U2 trigonal pyramids, and  a faceface with one Se(4)Nd4U square pyramid. In the fourth Se site, Se(4) is bonded to one U(1) and four equivalent Nd(1) atoms to form distorted SeNd4U square pyramids that share corners with four equivalent Se(3)Nd4U square pyramids, corners with four equivalent Se(4)Nd4U square pyramids,  a cornercorner with one Se(2)Nd4U trigonal bipyramid, corners with four equivalent Se(1)Nd2U2 trigonal pyramids,  an edgeedge with one Se(3)Nd4U square pyramid, edges with four equivalent Se(2)Nd4U trigonal bipyramids, edges with four equivalent Se(1)Nd2U2 trigonal pyramids, and  a faceface with one Se(3)Nd4U square pyramid.