Ni2P2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), one O(3), one O(4), one O(6), and one O(7) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and edges with two equivalent Ni(2)O6 octahedra. In the second Ni site, Ni(2) is bonded to one O(2), one O(3), one O(4), one O(6), and two equivalent O(5) atoms to form NiO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Ni(2)O6 octahedra, and edges with two equivalent Ni(1)O5 trigonal bipyramids. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Ni(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent Ni(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-54°. In the second P site, P(2) is bonded to one O(1), one O(2), one O(4), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Ni(2)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with three equivalent Ni(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 34-51°. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ni(2) and one P(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(2) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the fourth O site, O(1) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(2) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(2) atom. In the sixth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(1) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(2), and one P(2) atom.
Ni2P2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(2), one O(3), one O(4), one O(6), and one O(7) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and edges with two equivalent Ni(2)O6 octahedra. The Ni(1)-O(2) bond length is 2.03 Å. The Ni(1)-O(3) bond length is 2.02 Å. The Ni(1)-O(4) bond length is 2.05 Å. The Ni(1)-O(6) bond length is 2.00 Å. The Ni(1)-O(7) bond length is 1.96 Å. In the second Ni site, Ni(2) is bonded to one O(2), one O(3), one O(4), one O(6), and two equivalent O(5) atoms to form NiO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Ni(2)O6 octahedra, and edges with two equivalent Ni(1)O5 trigonal bipyramids. The Ni(2)-O(2) bond length is 2.05 Å. The Ni(2)-O(3) bond length is 2.04 Å. The Ni(2)-O(4) bond length is 2.16 Å. The Ni(2)-O(6) bond length is 2.09 Å. There is one shorter (2.03 Å) and one longer (2.10 Å) Ni(2)-O(5) bond length. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Ni(2)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent Ni(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 49-54°. The P(1)-O(1) bond length is 1.59 Å. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(5) bond length is 1.52 Å. The P(1)-O(7) bond length is 1.49 Å. In the second P site, P(2) is bonded to one O(1), one O(2), one O(4), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Ni(2)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with three equivalent Ni(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 34-51°. The P(2)-O(1) bond length is 1.57 Å. The P(2)-O(2) bond length is 1.53 Å. The P(2)-O(4) bond length is 1.52 Å. The P(2)-O(6) bond length is 1.52 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ni(2) and one P(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(2) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the fourth O site, O(1) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(2) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(2) atom. In the sixth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ni(1), one Ni(2), and one P(1) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(2), and one P(2) atom.