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NpNi2 is Cubic Laves structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Np is bonded in a 12-coordinate geometry to twelve equivalent Ni atoms. All Np–Ni bond lengths are 2.87 Å. Ni is bonded to six equivalent Np and six equivalent Ni atoms to form a mixture of corner, edge, and face-sharing NiNp6Ni6 cuboctahedra. All Ni–Ni bond lengths are 2.45 Å.
NpNi5 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Np is bonded in a distorted hexagonal planar geometry to eighteen Ni atoms. There are six shorter (2.79 Å) and twelve longer (3.12 Å) Np–Ni bond lengths. There are two inequivalent Ni sites. In the first Ni site, Ni is bonded to four equivalent Np and eight Ni atoms to form a mixture of edge, face, and corner-sharing NiNp4Ni8 cuboctahedra. There are four shorter (2.41 Å) and four longer (2.42 Å) Ni–Ni bond lengths. In the second Ni site, Ni is bonded to three equivalent Np and nine Ni atoms to form distorted NiNp3Ni9 cuboctahedra that share corners with twenty-one NiNp4Ni8 cuboctahedra, edges with six equivalent NiNp3Ni9 cuboctahedra, and faces with twenty-three NiNp4Ni8 cuboctahedra. All Ni–Ni bond lengths are 2.79 Å.