NaNpSeO6 crystallizes in the monoclinic C2/m space group. Na(1) is bonded to one O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted NaO5 square pyramids that share  a cornercorner with one Np(1)O7 pentagonal bipyramid,  an edgeedge with one Np(1)O7 pentagonal bipyramid, and edges with two equivalent Na(1)O5 square pyramids. Np(1) is bonded to one O(2), one O(3), one O(4), and four equivalent O(1) atoms to form distorted NpO7 pentagonal bipyramids that share  a cornercorner with one Na(1)O5 square pyramid, edges with two equivalent Np(1)O7 pentagonal bipyramids, and  an edgeedge with one Na(1)O5 square pyramid. Se(1) is bonded in a trigonal non-coplanar geometry to one O(3) and two equivalent O(1) atoms. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Np(1) and one Se(1) atom. In the second O site, O(2) is bonded in a single-bond geometry to one Np(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Np(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Na(1) and one Np(1) atom. In the fifth O site, O(5) is bonded in a water-like geometry to two equivalent Na(1) atoms.
NaNpSeO6 crystallizes in the monoclinic C2/m space group. Na(1) is bonded to one O(3), two equivalent O(4), and two equivalent O(5) atoms to form distorted NaO5 square pyramids that share  a cornercorner with one Np(1)O7 pentagonal bipyramid,  an edgeedge with one Np(1)O7 pentagonal bipyramid, and edges with two equivalent Na(1)O5 square pyramids. The Na(1)-O(3) bond length is 2.45 Å. There is one shorter (2.38 Å) and one longer (2.53 Å) Na(1)-O(4) bond length. Both Na(1)-O(5) bond lengths are 2.28 Å. Np(1) is bonded to one O(2), one O(3), one O(4), and four equivalent O(1) atoms to form distorted NpO7 pentagonal bipyramids that share  a cornercorner with one Na(1)O5 square pyramid, edges with two equivalent Np(1)O7 pentagonal bipyramids, and  an edgeedge with one Na(1)O5 square pyramid. The Np(1)-O(2) bond length is 1.79 Å. The Np(1)-O(3) bond length is 2.36 Å. The Np(1)-O(4) bond length is 1.83 Å. There are two shorter (2.39 Å) and two longer (2.57 Å) Np(1)-O(1) bond lengths. Se(1) is bonded in a trigonal non-coplanar geometry to one O(3) and two equivalent O(1) atoms. The Se(1)-O(3) bond length is 1.73 Å. Both Se(1)-O(1) bond lengths are 1.74 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Np(1) and one Se(1) atom. In the second O site, O(2) is bonded in a single-bond geometry to one Np(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Np(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to two equivalent Na(1) and one Np(1) atom. In the fifth O site, O(5) is bonded in a water-like geometry to two equivalent Na(1) atoms.