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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.
NpB6 is Calcium hexaboride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Np6+ is bonded in a 1-coordinate geometry to twenty-four equivalent B1- atoms. All Np–B bond lengths are 3.00 Å. B1- is bonded in a 1-coordinate geometry to four equivalent Np6+ and five equivalent B1- atoms. There is one shorter (1.62 Å) and four longer (1.75 Å) B–B bond length.
NpB4 crystallizes in the tetragonal P4/mbm space group. The structure is three-dimensional. Np6+ is bonded in a 12-coordinate geometry to eighteen B+1.50- atoms. There are a spread of Np–B bond distances ranging from 2.70–3.00 Å. There are three inequivalent B+1.50- sites. In the first B+1.50- site, B+1.50- is bonded in a distorted trigonal planar geometry to six equivalent Np6+ and three B+1.50- atoms. There is two shorter (1.69 Å) and one longer (1.74 Å) B–B bond length. In the second B+1.50- site, B+1.50- is bonded in a 1-coordinate geometry to four equivalent Np6+ and five B+1.50- atoms. There is one shorter (1.60 Å) and four longer (1.76 Å) B–B bond length. In the third B+1.50- site, B+1.50- is bonded in a 9-coordinate geometry to four equivalent Np6+ and five B+1.50- atoms. Both B–B bond lengths are 1.82 Å.