NpB2 is hexagonal omega structure structured and crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Np6+ is bonded to twelve equivalent B3- atoms to form a mixture of face and edge-sharing NpB12 cuboctahedra. All Np–B bond lengths are 2.66 Å. B3- is bonded in a 3-coordinate geometry to six equivalent Np6+ and three equivalent B3- atoms. All B–B bond lengths are 1.76 Å.