BaMgNiF7 crystallizes in the monoclinic P2_1/c space group. Ba(1) is bonded to one F(1), one F(2), two equivalent F(3), two equivalent F(4), two equivalent F(5), two equivalent F(6), and two equivalent F(7) atoms to form distorted BaF12 cuboctahedra that share corners with two equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Mg(1)F6 octahedra, corners with three equivalent Ni(1)F6 octahedra, edges with four equivalent Ba(1)F12 cuboctahedra, edges with two equivalent Mg(1)F6 octahedra, edges with two equivalent Ni(1)F6 octahedra,  a faceface with one Mg(1)F6 octahedra, and  a faceface with one Ni(1)F6 octahedra. The corner-sharing octahedral tilt angles range from 43-52°. Mg(1) is bonded to one F(1), one F(2), one F(3), one F(5), one F(6), and one F(7) atom to form MgF6 octahedra that share corners with three equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Ni(1)F6 octahedra, edges with two equivalent Ba(1)F12 cuboctahedra,  an edgeedge with one Ni(1)F6 octahedra, and  a faceface with one Ba(1)F12 cuboctahedra. The corner-sharing octahedral tilt angles range from 46-52°. Ni(1) is bonded to one F(1), one F(2), one F(3), one F(4), one F(5), and one F(6) atom to form NiF6 octahedra that share corners with three equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Mg(1)F6 octahedra, edges with two equivalent Ba(1)F12 cuboctahedra,  an edgeedge with one Mg(1)F6 octahedra, and  a faceface with one Ba(1)F12 cuboctahedra. The corner-sharing octahedral tilt angles range from 46-52°. There are seven inequivalent F sites. In the first F site, F(1) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Mg(1), and one Ni(1) atom. In the second F site, F(2) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Mg(1), and one Ni(1) atom. In the third F site, F(3) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the fourth F site, F(4) is bonded in a distorted single-bond geometry to two equivalent Ba(1) and one Ni(1) atom. In the fifth F site, F(5) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the sixth F site, F(6) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the seventh F site, F(7) is bonded in a distorted trigonal planar geometry to two equivalent Ba(1) and one Mg(1) atom.
BaMgNiF7 crystallizes in the monoclinic P2_1/c space group. Ba(1) is bonded to one F(1), one F(2), two equivalent F(3), two equivalent F(4), two equivalent F(5), two equivalent F(6), and two equivalent F(7) atoms to form distorted BaF12 cuboctahedra that share corners with two equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Mg(1)F6 octahedra, corners with three equivalent Ni(1)F6 octahedra, edges with four equivalent Ba(1)F12 cuboctahedra, edges with two equivalent Mg(1)F6 octahedra, edges with two equivalent Ni(1)F6 octahedra,  a faceface with one Mg(1)F6 octahedra, and  a faceface with one Ni(1)F6 octahedra. The corner-sharing octahedral tilt angles range from 43-52°. The Ba(1)-F(1) bond length is 3.05 Å. The Ba(1)-F(2) bond length is 3.06 Å. There is one shorter (2.92 Å) and one longer (2.96 Å) Ba(1)-F(3) bond length. There is one shorter (2.74 Å) and one longer (2.80 Å) Ba(1)-F(4) bond length. There is one shorter (3.00 Å) and one longer (3.20 Å) Ba(1)-F(5) bond length. There is one shorter (2.98 Å) and one longer (3.00 Å) Ba(1)-F(6) bond length. There is one shorter (2.62 Å) and one longer (2.65 Å) Ba(1)-F(7) bond length. Mg(1) is bonded to one F(1), one F(2), one F(3), one F(5), one F(6), and one F(7) atom to form MgF6 octahedra that share corners with three equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Ni(1)F6 octahedra, edges with two equivalent Ba(1)F12 cuboctahedra,  an edgeedge with one Ni(1)F6 octahedra, and  a faceface with one Ba(1)F12 cuboctahedra. The corner-sharing octahedral tilt angles range from 46-52°. The Mg(1)-F(1) bond length is 1.97 Å. The Mg(1)-F(2) bond length is 2.02 Å. The Mg(1)-F(3) bond length is 2.13 Å. The Mg(1)-F(5) bond length is 2.04 Å. The Mg(1)-F(6) bond length is 2.06 Å. The Mg(1)-F(7) bond length is 1.92 Å. Ni(1) is bonded to one F(1), one F(2), one F(3), one F(4), one F(5), and one F(6) atom to form NiF6 octahedra that share corners with three equivalent Ba(1)F12 cuboctahedra, corners with three equivalent Mg(1)F6 octahedra, edges with two equivalent Ba(1)F12 cuboctahedra,  an edgeedge with one Mg(1)F6 octahedra, and  a faceface with one Ba(1)F12 cuboctahedra. The corner-sharing octahedral tilt angles range from 46-52°. The Ni(1)-F(1) bond length is 1.93 Å. The Ni(1)-F(2) bond length is 1.93 Å. The Ni(1)-F(3) bond length is 1.95 Å. The Ni(1)-F(4) bond length is 1.85 Å. The Ni(1)-F(5) bond length is 1.93 Å. The Ni(1)-F(6) bond length is 1.98 Å. There are seven inequivalent F sites. In the first F site, F(1) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Mg(1), and one Ni(1) atom. In the second F site, F(2) is bonded in a distorted bent 120 degrees geometry to one Ba(1), one Mg(1), and one Ni(1) atom. In the third F site, F(3) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the fourth F site, F(4) is bonded in a distorted single-bond geometry to two equivalent Ba(1) and one Ni(1) atom. In the fifth F site, F(5) is bonded in a 2-coordinate geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the sixth F site, F(6) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Mg(1), and one Ni(1) atom. In the seventh F site, F(7) is bonded in a distorted trigonal planar geometry to two equivalent Ba(1) and one Mg(1) atom.