Np2Co17 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Np sites. In the first Np site, Np(1) is bonded in a 18-coordinate geometry to six equivalent Co(3) and twelve equivalent Co(4) atoms. In the second Np site, Np(2) is bonded in a 20-coordinate geometry to two equivalent Co(1), six equivalent Co(2), six equivalent Co(3), and six equivalent Co(4) atoms. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded in a 14-coordinate geometry to one Np(2), one Co(1), three equivalent Co(2), three equivalent Co(4), and six equivalent Co(3) atoms. In the second Co site, Co(2) is bonded to two equivalent Np(2), two equivalent Co(1), four equivalent Co(3), and four equivalent Co(4) atoms to form CoNp2Co10 cuboctahedra that share corners with four equivalent Co(2)Np2Co10 cuboctahedra, corners with ten equivalent Co(4)Np3Co9 cuboctahedra, edges with six equivalent Co(4)Np3Co9 cuboctahedra, faces with four equivalent Co(4)Np3Co9 cuboctahedra, and faces with six equivalent Co(2)Np2Co10 cuboctahedra. In the third Co site, Co(3) is bonded in a 12-coordinate geometry to one Np(1), one Np(2), two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), and four equivalent Co(4) atoms. In the fourth Co site, Co(4) is bonded to one Np(2), two equivalent Np(1), one Co(1), two equivalent Co(2), two equivalent Co(4), and four equivalent Co(3) atoms to form distorted CoNp3Co9 cuboctahedra that share corners with five equivalent Co(2)Np2Co10 cuboctahedra, corners with ten equivalent Co(4)Np3Co9 cuboctahedra, edges with three equivalent Co(2)Np2Co10 cuboctahedra, edges with five equivalent Co(4)Np3Co9 cuboctahedra, faces with two equivalent Co(2)Np2Co10 cuboctahedra, and faces with eight equivalent Co(4)Np3Co9 cuboctahedra.
Np2Co17 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Np sites. In the first Np site, Np(1) is bonded in a 18-coordinate geometry to six equivalent Co(3) and twelve equivalent Co(4) atoms. All Np(1)-Co(3) bond lengths are 2.80 Å. There are six shorter (2.90 Å) and six longer (3.16 Å) Np(1)-Co(4) bond lengths. In the second Np site, Np(2) is bonded in a 20-coordinate geometry to two equivalent Co(1), six equivalent Co(2), six equivalent Co(3), and six equivalent Co(4) atoms. Both Np(2)-Co(1) bond lengths are 2.75 Å. All Np(2)-Co(2) bond lengths are 3.05 Å. All Np(2)-Co(3) bond lengths are 2.90 Å. All Np(2)-Co(4) bond lengths are 2.94 Å. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded in a 14-coordinate geometry to one Np(2), one Co(1), three equivalent Co(2), three equivalent Co(4), and six equivalent Co(3) atoms. The Co(1)-Co(1) bond length is 2.35 Å. All Co(1)-Co(2) bond lengths are 2.46 Å. All Co(1)-Co(4) bond lengths are 2.56 Å. All Co(1)-Co(3) bond lengths are 2.67 Å. In the second Co site, Co(2) is bonded to two equivalent Np(2), two equivalent Co(1), four equivalent Co(3), and four equivalent Co(4) atoms to form CoNp2Co10 cuboctahedra that share corners with four equivalent Co(2)Np2Co10 cuboctahedra, corners with ten equivalent Co(4)Np3Co9 cuboctahedra, edges with six equivalent Co(4)Np3Co9 cuboctahedra, faces with four equivalent Co(4)Np3Co9 cuboctahedra, and faces with six equivalent Co(2)Np2Co10 cuboctahedra. All Co(2)-Co(3) bond lengths are 2.35 Å. All Co(2)-Co(4) bond lengths are 2.35 Å. In the third Co site, Co(3) is bonded in a 12-coordinate geometry to one Np(1), one Np(2), two equivalent Co(1), two equivalent Co(2), two equivalent Co(3), and four equivalent Co(4) atoms. There is one shorter (2.29 Å) and one longer (2.51 Å) Co(3)-Co(3) bond length. There are two shorter (2.41 Å) and two longer (2.46 Å) Co(3)-Co(4) bond lengths. In the fourth Co site, Co(4) is bonded to one Np(2), two equivalent Np(1), one Co(1), two equivalent Co(2), two equivalent Co(4), and four equivalent Co(3) atoms to form distorted CoNp3Co9 cuboctahedra that share corners with five equivalent Co(2)Np2Co10 cuboctahedra, corners with ten equivalent Co(4)Np3Co9 cuboctahedra, edges with three equivalent Co(2)Np2Co10 cuboctahedra, edges with five equivalent Co(4)Np3Co9 cuboctahedra, faces with two equivalent Co(2)Np2Co10 cuboctahedra, and faces with eight equivalent Co(4)Np3Co9 cuboctahedra. Both Co(4)-Co(4) bond lengths are 2.34 Å.