Mn2Ni9N8 crystallizes in the monoclinic C2/m space group. Mn(1) is bonded in a trigonal planar geometry to one N(1), one N(2), and one N(3) atom. There are five inequivalent Ni sites. In the first Ni site, Ni(1) is bonded in a linear geometry to two equivalent N(4) atoms. In the second Ni site, Ni(2) is bonded in a distorted rectangular see-saw-like geometry to one N(1), one N(2), and two equivalent N(4) atoms. In the third Ni site, Ni(3) is bonded in a see-saw-like geometry to one N(3), one N(4), and two equivalent N(2) atoms. In the fourth Ni site, Ni(4) is bonded to one N(3), two equivalent N(1), and two equivalent N(4) atoms to form a mixture of distorted edge and corner-sharing NiN5 square pyramids. In the fifth Ni site, Ni(5) is bonded to one N(1), one N(2), and three equivalent N(3) atoms to form a mixture of distorted edge and corner-sharing NiN5 square pyramids. There are four inequivalent N sites. In the first N site, N(1) is bonded in a 5-coordinate geometry to one Mn(1), one Ni(2), one Ni(5), and two equivalent Ni(4) atoms. In the second N site, N(2) is bonded to one Mn(1), one Ni(2), one Ni(5), and two equivalent Ni(3) atoms to form NMnNi4 trigonal bipyramids that share corners with two equivalent N(3)MnNi5 octahedra, corners with two equivalent N(2)MnNi4 trigonal bipyramids, edges with two equivalent N(3)MnNi5 octahedra, and edges with two equivalent N(4)Ni6 octahedra. The corner-sharing octahedral tilt angles range from 6-52°. In the third N site, N(3) is bonded to one Mn(1), one Ni(3), one Ni(4), and three equivalent Ni(5) atoms to form distorted NMnNi5 octahedra that share corners with two equivalent N(3)MnNi5 octahedra, corners with three equivalent N(4)Ni6 octahedra, corners with two equivalent N(2)MnNi4 trigonal bipyramids, edges with two equivalent N(3)MnNi5 octahedra, and edges with two equivalent N(2)MnNi4 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 10-62°. In the fourth N site, N(4) is bonded to one Ni(1), one Ni(3), two equivalent Ni(2), and two equivalent Ni(4) atoms to form NNi6 octahedra that share corners with three equivalent N(3)MnNi5 octahedra, corners with three equivalent N(4)Ni6 octahedra,  an edgeedge with one N(4)Ni6 octahedra, and edges with two equivalent N(2)MnNi4 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-62°.
Mn2Ni9N8 crystallizes in the monoclinic C2/m space group. Mn(1) is bonded in a trigonal planar geometry to one N(1), one N(2), and one N(3) atom. The Mn(1)-N(1) bond length is 1.75 Å. The Mn(1)-N(2) bond length is 1.76 Å. The Mn(1)-N(3) bond length is 1.78 Å. There are five inequivalent Ni sites. In the first Ni site, Ni(1) is bonded in a linear geometry to two equivalent N(4) atoms. Both Ni(1)-N(4) bond lengths are 1.79 Å. In the second Ni site, Ni(2) is bonded in a distorted rectangular see-saw-like geometry to one N(1), one N(2), and two equivalent N(4) atoms. The Ni(2)-N(1) bond length is 1.91 Å. The Ni(2)-N(2) bond length is 1.94 Å. Both Ni(2)-N(4) bond lengths are 2.11 Å. In the third Ni site, Ni(3) is bonded in a see-saw-like geometry to one N(3), one N(4), and two equivalent N(2) atoms. The Ni(3)-N(3) bond length is 2.02 Å. The Ni(3)-N(4) bond length is 1.95 Å. Both Ni(3)-N(2) bond lengths are 2.08 Å. In the fourth Ni site, Ni(4) is bonded to one N(3), two equivalent N(1), and two equivalent N(4) atoms to form a mixture of distorted edge and corner-sharing NiN5 square pyramids. The Ni(4)-N(3) bond length is 2.07 Å. Both Ni(4)-N(1) bond lengths are 2.10 Å. There is one shorter (1.95 Å) and one longer (2.10 Å) Ni(4)-N(4) bond length. In the fifth Ni site, Ni(5) is bonded to one N(1), one N(2), and three equivalent N(3) atoms to form a mixture of distorted edge and corner-sharing NiN5 square pyramids. The Ni(5)-N(1) bond length is 2.04 Å. The Ni(5)-N(2) bond length is 1.91 Å. There are two shorter (2.08 Å) and one longer (2.14 Å) Ni(5)-N(3) bond length. There are four inequivalent N sites. In the first N site, N(1) is bonded in a 5-coordinate geometry to one Mn(1), one Ni(2), one Ni(5), and two equivalent Ni(4) atoms. In the second N site, N(2) is bonded to one Mn(1), one Ni(2), one Ni(5), and two equivalent Ni(3) atoms to form NMnNi4 trigonal bipyramids that share corners with two equivalent N(3)MnNi5 octahedra, corners with two equivalent N(2)MnNi4 trigonal bipyramids, edges with two equivalent N(3)MnNi5 octahedra, and edges with two equivalent N(4)Ni6 octahedra. The corner-sharing octahedral tilt angles range from 6-52°. In the third N site, N(3) is bonded to one Mn(1), one Ni(3), one Ni(4), and three equivalent Ni(5) atoms to form distorted NMnNi5 octahedra that share corners with two equivalent N(3)MnNi5 octahedra, corners with three equivalent N(4)Ni6 octahedra, corners with two equivalent N(2)MnNi4 trigonal bipyramids, edges with two equivalent N(3)MnNi5 octahedra, and edges with two equivalent N(2)MnNi4 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 10-62°. In the fourth N site, N(4) is bonded to one Ni(1), one Ni(3), two equivalent Ni(2), and two equivalent Ni(4) atoms to form NNi6 octahedra that share corners with three equivalent N(3)MnNi5 octahedra, corners with three equivalent N(4)Ni6 octahedra,  an edgeedge with one N(4)Ni6 octahedra, and edges with two equivalent N(2)MnNi4 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-62°.