Mg5(C2O9)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(3), one O(6), one O(8), one O(9), and two equivalent O(1) atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 54-66°. In the second Mg site, Mg(2) is bonded to one O(2), one O(3), one O(6), one O(9), and two equivalent O(4) atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 53-66°. In the third Mg site, Mg(3) is bonded to two equivalent O(5), two equivalent O(7), and two equivalent O(9) atoms to form corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 53-54°. There are two inequivalent C sites. In the first C site, C(1) is bonded in a trigonal planar geometry to one O(1), one O(4), and one O(6) atom. In the second C site, C(2) is bonded in a trigonal planar geometry to one O(3), one O(5), and one O(7) atom. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 150 degrees geometry to one Mg(3) and one C(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(2), and one C(1) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Mg(3) and one C(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Mg(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one C(1) atom. In the sixth O site, O(9) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one Mg(3) atom. In the seventh O site, O(2) is bonded in a single-bond geometry to one Mg(2) atom. In the eighth O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one C(2) atom. In the ninth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Mg(2) and one C(1) atom.
Mg5(C2O9)2 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to one O(3), one O(6), one O(8), one O(9), and two equivalent O(1) atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 54-66°. The Mg(1)-O(3) bond length is 2.09 Å. The Mg(1)-O(6) bond length is 2.12 Å. The Mg(1)-O(8) bond length is 2.17 Å. The Mg(1)-O(9) bond length is 2.04 Å. There is one shorter (2.07 Å) and one longer (2.11 Å) Mg(1)-O(1) bond length. In the second Mg site, Mg(2) is bonded to one O(2), one O(3), one O(6), one O(9), and two equivalent O(4) atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 53-66°. The Mg(2)-O(2) bond length is 2.17 Å. The Mg(2)-O(3) bond length is 2.14 Å. The Mg(2)-O(6) bond length is 2.11 Å. The Mg(2)-O(9) bond length is 2.04 Å. There is one shorter (2.08 Å) and one longer (2.13 Å) Mg(2)-O(4) bond length. In the third Mg site, Mg(3) is bonded to two equivalent O(5), two equivalent O(7), and two equivalent O(9) atoms to form corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles range from 53-54°. Both Mg(3)-O(5) bond lengths are 2.02 Å. Both Mg(3)-O(7) bond lengths are 2.11 Å. Both Mg(3)-O(9) bond lengths are 2.01 Å. There are two inequivalent C sites. In the first C site, C(1) is bonded in a trigonal planar geometry to one O(1), one O(4), and one O(6) atom. The C(1)-O(1) bond length is 1.30 Å. The C(1)-O(4) bond length is 1.30 Å. The C(1)-O(6) bond length is 1.28 Å. In the second C site, C(2) is bonded in a trigonal planar geometry to one O(3), one O(5), and one O(7) atom. The C(2)-O(3) bond length is 1.31 Å. The C(2)-O(5) bond length is 1.27 Å. The C(2)-O(7) bond length is 1.29 Å. There are nine inequivalent O sites. In the first O site, O(5) is bonded in a bent 150 degrees geometry to one Mg(3) and one C(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Mg(1), one Mg(2), and one C(1) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Mg(3) and one C(2) atom. In the fourth O site, O(8) is bonded in a single-bond geometry to one Mg(1) atom. In the fifth O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one C(1) atom. In the sixth O site, O(9) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one Mg(3) atom. In the seventh O site, O(2) is bonded in a single-bond geometry to one Mg(2) atom. In the eighth O site, O(3) is bonded in a trigonal planar geometry to one Mg(1), one Mg(2), and one C(2) atom. In the ninth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Mg(2) and one C(1) atom.