Hf8Ni21 crystallizes in the triclinic P-1 space group. There are four inequivalent Hf sites. In the first Hf site, Hf(1) is bonded in a 12-coordinate geometry to one Ni(11), one Ni(5), one Ni(6), one Ni(7), two equivalent Ni(2), two equivalent Ni(3), two equivalent Ni(8), and two equivalent Ni(9) atoms. In the second Hf site, Hf(2) is bonded in a 13-coordinate geometry to one Ni(1), one Ni(2), one Ni(3), one Ni(5), one Ni(7), one Ni(8), one Ni(9), two equivalent Ni(10), two equivalent Ni(11), and two equivalent Ni(4) atoms. In the third Hf site, Hf(3) is bonded in a 12-coordinate geometry to one Ni(1), one Ni(11), one Ni(3), one Ni(4), one Ni(6), one Ni(7), one Ni(8), one Ni(9), two equivalent Ni(10), and two equivalent Ni(5) atoms. In the fourth Hf site, Hf(4) is bonded in a 10-coordinate geometry to one Ni(10), one Ni(2), one Ni(3), one Ni(4), one Ni(5), one Ni(9), two equivalent Ni(6), and two equivalent Ni(7) atoms. There are eleven inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent Hf(2), two equivalent Hf(3), two equivalent Ni(11), two equivalent Ni(3), two equivalent Ni(8), and two equivalent Ni(9) atoms to form face-sharing NiHf4Ni8 cuboctahedra. In the second Ni site, Ni(2) is bonded in a 10-coordinate geometry to one Hf(2), one Hf(4), two equivalent Hf(1), one Ni(4), one Ni(5), one Ni(6), one Ni(7), one Ni(8), and one Ni(9) atom. In the third Ni site, Ni(3) is bonded to one Hf(2), one Hf(3), one Hf(4), two equivalent Hf(1), one Ni(1), one Ni(11), one Ni(4), one Ni(6), one Ni(7), one Ni(8), and one Ni(9) atom to form distorted NiHf5Ni7 cuboctahedra that share  a cornercorner with one Ni(3)Hf5Ni7 cuboctahedra,  an edgeedge with one Ni(3)Hf5Ni7 cuboctahedra,  a faceface with one Ni(1)Hf4Ni8 cuboctahedra, and faces with three equivalent Ni(9)Hf5Ni7 cuboctahedra. In the fourth Ni site, Ni(4) is bonded in a 3-coordinate geometry to one Hf(3), one Hf(4), two equivalent Hf(2), one Ni(2), one Ni(3), one Ni(5), one Ni(7), and two equivalent Ni(6) atoms. In the fifth Ni site, Ni(5) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(2), one Hf(4), two equivalent Hf(3), one Ni(11), one Ni(2), one Ni(4), one Ni(6), one Ni(9), and two equivalent Ni(10) atoms. In the sixth Ni site, Ni(6) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(3), two equivalent Hf(4), one Ni(2), one Ni(3), one Ni(5), one Ni(6), one Ni(7), one Ni(8), and two equivalent Ni(4) atoms. In the seventh Ni site, Ni(7) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(2), one Hf(3), two equivalent Hf(4), one Ni(10), one Ni(11), one Ni(2), one Ni(3), one Ni(4), one Ni(6), and one Ni(8) atom. In the eighth Ni site, Ni(8) is bonded in a 4-coordinate geometry to one Hf(2), one Hf(3), two equivalent Hf(1), one Ni(1), one Ni(2), one Ni(3), one Ni(6), one Ni(7), one Ni(9), and two equivalent Ni(11) atoms. In the ninth Ni site, Ni(9) is bonded to one Hf(2), one Hf(3), one Hf(4), two equivalent Hf(1), one Ni(1), one Ni(10), one Ni(11), one Ni(2), one Ni(3), one Ni(5), and one Ni(8) atom to form distorted NiHf5Ni7 cuboctahedra that share  a cornercorner with one Ni(9)Hf5Ni7 cuboctahedra,  an edgeedge with one Ni(9)Hf5Ni7 cuboctahedra,  a faceface with one Ni(1)Hf4Ni8 cuboctahedra, and faces with three equivalent Ni(3)Hf5Ni7 cuboctahedra. In the tenth Ni site, Ni(10) is bonded in a 5-coordinate geometry to one Hf(4), two equivalent Hf(2), two equivalent Hf(3), one Ni(7), one Ni(9), and two equivalent Ni(5) atoms. In the eleventh Ni site, Ni(11) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(3), two equivalent Hf(2), one Ni(1), one Ni(11), one Ni(3), one Ni(5), one Ni(7), one Ni(9), and two equivalent Ni(8) atoms.
Hf8Ni21 crystallizes in the triclinic P-1 space group. There are four inequivalent Hf sites. In the first Hf site, Hf(1) is bonded in a 12-coordinate geometry to one Ni(11), one Ni(5), one Ni(6), one Ni(7), two equivalent Ni(2), two equivalent Ni(3), two equivalent Ni(8), and two equivalent Ni(9) atoms. The Hf(1)-Ni(11) bond length is 2.62 Å. The Hf(1)-Ni(5) bond length is 2.74 Å. The Hf(1)-Ni(6) bond length is 2.84 Å. The Hf(1)-Ni(7) bond length is 2.91 Å. There is one shorter (2.64 Å) and one longer (2.66 Å) Hf(1)-Ni(2) bond length. There is one shorter (2.78 Å) and one longer (2.82 Å) Hf(1)-Ni(3) bond length. There is one shorter (2.64 Å) and one longer (2.71 Å) Hf(1)-Ni(8) bond length. There is one shorter (2.76 Å) and one longer (2.80 Å) Hf(1)-Ni(9) bond length. In the second Hf site, Hf(2) is bonded in a 13-coordinate geometry to one Ni(1), one Ni(2), one Ni(3), one Ni(5), one Ni(7), one Ni(8), one Ni(9), two equivalent Ni(10), two equivalent Ni(11), and two equivalent Ni(4) atoms. The Hf(2)-Ni(1) bond length is 2.78 Å. The Hf(2)-Ni(2) bond length is 2.67 Å. The Hf(2)-Ni(3) bond length is 2.95 Å. The Hf(2)-Ni(5) bond length is 2.69 Å. The Hf(2)-Ni(7) bond length is 2.76 Å. The Hf(2)-Ni(8) bond length is 2.63 Å. The Hf(2)-Ni(9) bond length is 2.77 Å. There is one shorter (2.61 Å) and one longer (2.66 Å) Hf(2)-Ni(10) bond length. There is one shorter (2.67 Å) and one longer (2.70 Å) Hf(2)-Ni(11) bond length. There is one shorter (2.67 Å) and one longer (3.10 Å) Hf(2)-Ni(4) bond length. In the third Hf site, Hf(3) is bonded in a 12-coordinate geometry to one Ni(1), one Ni(11), one Ni(3), one Ni(4), one Ni(6), one Ni(7), one Ni(8), one Ni(9), two equivalent Ni(10), and two equivalent Ni(5) atoms. The Hf(3)-Ni(1) bond length is 2.77 Å. The Hf(3)-Ni(11) bond length is 2.66 Å. The Hf(3)-Ni(3) bond length is 2.86 Å. The Hf(3)-Ni(4) bond length is 2.67 Å. The Hf(3)-Ni(6) bond length is 2.68 Å. The Hf(3)-Ni(7) bond length is 2.67 Å. The Hf(3)-Ni(8) bond length is 2.66 Å. The Hf(3)-Ni(9) bond length is 2.85 Å. There is one shorter (2.59 Å) and one longer (2.65 Å) Hf(3)-Ni(10) bond length. There is one shorter (2.68 Å) and one longer (2.73 Å) Hf(3)-Ni(5) bond length. In the fourth Hf site, Hf(4) is bonded in a 10-coordinate geometry to one Ni(10), one Ni(2), one Ni(3), one Ni(4), one Ni(5), one Ni(9), two equivalent Ni(6), and two equivalent Ni(7) atoms. The Hf(4)-Ni(10) bond length is 2.59 Å. The Hf(4)-Ni(2) bond length is 2.66 Å. The Hf(4)-Ni(3) bond length is 2.70 Å. The Hf(4)-Ni(4) bond length is 2.64 Å. The Hf(4)-Ni(5) bond length is 2.63 Å. The Hf(4)-Ni(9) bond length is 2.64 Å. There is one shorter (2.61 Å) and one longer (2.64 Å) Hf(4)-Ni(6) bond length. There is one shorter (2.62 Å) and one longer (2.76 Å) Hf(4)-Ni(7) bond length. There are eleven inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent Hf(2), two equivalent Hf(3), two equivalent Ni(11), two equivalent Ni(3), two equivalent Ni(8), and two equivalent Ni(9) atoms to form face-sharing NiHf4Ni8 cuboctahedra. Both Ni(1)-Ni(11) bond lengths are 2.50 Å. Both Ni(1)-Ni(3) bond lengths are 2.41 Å. Both Ni(1)-Ni(8) bond lengths are 2.58 Å. Both Ni(1)-Ni(9) bond lengths are 2.43 Å. In the second Ni site, Ni(2) is bonded in a 10-coordinate geometry to one Hf(2), one Hf(4), two equivalent Hf(1), one Ni(4), one Ni(5), one Ni(6), one Ni(7), one Ni(8), and one Ni(9) atom. The Ni(2)-Ni(4) bond length is 2.46 Å. The Ni(2)-Ni(5) bond length is 2.67 Å. The Ni(2)-Ni(6) bond length is 2.38 Å. The Ni(2)-Ni(7) bond length is 2.67 Å. The Ni(2)-Ni(8) bond length is 2.65 Å. The Ni(2)-Ni(9) bond length is 2.52 Å. In the third Ni site, Ni(3) is bonded to one Hf(2), one Hf(3), one Hf(4), two equivalent Hf(1), one Ni(1), one Ni(11), one Ni(4), one Ni(6), one Ni(7), one Ni(8), and one Ni(9) atom to form distorted NiHf5Ni7 cuboctahedra that share  a cornercorner with one Ni(3)Hf5Ni7 cuboctahedra,  an edgeedge with one Ni(3)Hf5Ni7 cuboctahedra,  a faceface with one Ni(1)Hf4Ni8 cuboctahedra, and faces with three equivalent Ni(9)Hf5Ni7 cuboctahedra. The Ni(3)-Ni(11) bond length is 2.49 Å. The Ni(3)-Ni(4) bond length is 2.49 Å. The Ni(3)-Ni(6) bond length is 2.48 Å. The Ni(3)-Ni(7) bond length is 2.50 Å. The Ni(3)-Ni(8) bond length is 2.53 Å. The Ni(3)-Ni(9) bond length is 2.43 Å. In the fourth Ni site, Ni(4) is bonded in a 3-coordinate geometry to one Hf(3), one Hf(4), two equivalent Hf(2), one Ni(2), one Ni(3), one Ni(5), one Ni(7), and two equivalent Ni(6) atoms. The Ni(4)-Ni(5) bond length is 2.42 Å. The Ni(4)-Ni(7) bond length is 2.56 Å. There is one shorter (2.74 Å) and one longer (2.84 Å) Ni(4)-Ni(6) bond length. In the fifth Ni site, Ni(5) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(2), one Hf(4), two equivalent Hf(3), one Ni(11), one Ni(2), one Ni(4), one Ni(6), one Ni(9), and two equivalent Ni(10) atoms. The Ni(5)-Ni(11) bond length is 2.71 Å. The Ni(5)-Ni(6) bond length is 2.73 Å. The Ni(5)-Ni(9) bond length is 2.53 Å. There is one shorter (2.77 Å) and one longer (2.79 Å) Ni(5)-Ni(10) bond length. In the sixth Ni site, Ni(6) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(3), two equivalent Hf(4), one Ni(2), one Ni(3), one Ni(5), one Ni(6), one Ni(7), one Ni(8), and two equivalent Ni(4) atoms. The Ni(6)-Ni(6) bond length is 2.50 Å. The Ni(6)-Ni(7) bond length is 2.70 Å. The Ni(6)-Ni(8) bond length is 2.65 Å. In the seventh Ni site, Ni(7) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(2), one Hf(3), two equivalent Hf(4), one Ni(10), one Ni(11), one Ni(2), one Ni(3), one Ni(4), one Ni(6), and one Ni(8) atom. The Ni(7)-Ni(10) bond length is 2.66 Å. The Ni(7)-Ni(11) bond length is 2.74 Å. The Ni(7)-Ni(8) bond length is 2.46 Å. In the eighth Ni site, Ni(8) is bonded in a 4-coordinate geometry to one Hf(2), one Hf(3), two equivalent Hf(1), one Ni(1), one Ni(2), one Ni(3), one Ni(6), one Ni(7), one Ni(9), and two equivalent Ni(11) atoms. The Ni(8)-Ni(9) bond length is 2.57 Å. There is one shorter (2.71 Å) and one longer (2.82 Å) Ni(8)-Ni(11) bond length. In the ninth Ni site, Ni(9) is bonded to one Hf(2), one Hf(3), one Hf(4), two equivalent Hf(1), one Ni(1), one Ni(10), one Ni(11), one Ni(2), one Ni(3), one Ni(5), and one Ni(8) atom to form distorted NiHf5Ni7 cuboctahedra that share  a cornercorner with one Ni(9)Hf5Ni7 cuboctahedra,  an edgeedge with one Ni(9)Hf5Ni7 cuboctahedra,  a faceface with one Ni(1)Hf4Ni8 cuboctahedra, and faces with three equivalent Ni(3)Hf5Ni7 cuboctahedra. The Ni(9)-Ni(10) bond length is 2.58 Å. The Ni(9)-Ni(11) bond length is 2.54 Å. In the tenth Ni site, Ni(10) is bonded in a 5-coordinate geometry to one Hf(4), two equivalent Hf(2), two equivalent Hf(3), one Ni(7), one Ni(9), and two equivalent Ni(5) atoms. In the eleventh Ni site, Ni(11) is bonded in a 12-coordinate geometry to one Hf(1), one Hf(3), two equivalent Hf(2), one Ni(1), one Ni(11), one Ni(3), one Ni(5), one Ni(7), one Ni(9), and two equivalent Ni(8) atoms. The Ni(11)-Ni(11) bond length is 2.44 Å.