MgFe4(HO2)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(4), one O(8), and two equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share  a cornercorner with one Fe(4)O6 octahedra, corners with four equivalent Fe(2)O5 square pyramids, and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 49°. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(6), two equivalent O(3), and two equivalent O(7) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Fe(2)O5 square pyramid, edges with two equivalent Fe(4)O6 octahedra, and edges with two equivalent Fe(1)O5 trigonal bipyramids. In the second Fe site, Fe(2) is bonded to one O(6), two equivalent O(4), and two equivalent O(8) atoms to form distorted FeO5 square pyramids that share  a cornercorner with one Fe(1)O5 trigonal bipyramid, corners with four equivalent Mg(1)O4 trigonal pyramids, edges with two equivalent Fe(4)O6 octahedra, and edges with two equivalent Fe(2)O5 square pyramids. In the third Fe site, Fe(3) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(1), and two equivalent O(5) atoms. In the fourth Fe site, Fe(4) is bonded to one O(3), one O(8), two equivalent O(2), and two equivalent O(6) atoms to form FeO6 octahedra that share  a cornercorner with one Mg(1)O4 trigonal pyramid, edges with two equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(2)O5 square pyramids, and edges with two equivalent Fe(1)O5 trigonal bipyramids. There are four inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(5) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(2) atom. In the third H site, H(3) is bonded in a single-bond geometry to one O(3) atom. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(4) atom. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Fe(3) atoms. In the second O site, O(2) is bonded in a distorted single-bond geometry to two equivalent Fe(4) and one H(2) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Fe(4), two equivalent Fe(1), and one H(3) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Mg(1), two equivalent Fe(2), and one H(4) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to two equivalent Fe(3) and one H(1) atom. In the sixth O site, O(6) is bonded in a rectangular see-saw-like geometry to one Fe(1), one Fe(2), and two equivalent Fe(4) atoms. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the eighth O site, O(8) is bonded to one Mg(1), one Fe(4), and two equivalent Fe(2) atoms to form distorted corner-sharing OMgFe3 tetrahedra.
MgFe4(HO2)4 crystallizes in the monoclinic Pm space group. Mg(1) is bonded to one O(4), one O(8), and two equivalent O(1) atoms to form distorted MgO4 trigonal pyramids that share  a cornercorner with one Fe(4)O6 octahedra, corners with four equivalent Fe(2)O5 square pyramids, and corners with two equivalent Mg(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles are 49°. The Mg(1)-O(4) bond length is 1.98 Å. The Mg(1)-O(8) bond length is 1.91 Å. Both Mg(1)-O(1) bond lengths are 2.06 Å. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(6), two equivalent O(3), and two equivalent O(7) atoms to form distorted FeO5 trigonal bipyramids that share  a cornercorner with one Fe(2)O5 square pyramid, edges with two equivalent Fe(4)O6 octahedra, and edges with two equivalent Fe(1)O5 trigonal bipyramids. The Fe(1)-O(6) bond length is 2.10 Å. Both Fe(1)-O(3) bond lengths are 2.14 Å. Both Fe(1)-O(7) bond lengths are 2.09 Å. In the second Fe site, Fe(2) is bonded to one O(6), two equivalent O(4), and two equivalent O(8) atoms to form distorted FeO5 square pyramids that share  a cornercorner with one Fe(1)O5 trigonal bipyramid, corners with four equivalent Mg(1)O4 trigonal pyramids, edges with two equivalent Fe(4)O6 octahedra, and edges with two equivalent Fe(2)O5 square pyramids. The Fe(2)-O(6) bond length is 2.22 Å. Both Fe(2)-O(4) bond lengths are 2.10 Å. Both Fe(2)-O(8) bond lengths are 2.04 Å. In the third Fe site, Fe(3) is bonded in a 5-coordinate geometry to one O(7), two equivalent O(1), and two equivalent O(5) atoms. The Fe(3)-O(7) bond length is 1.83 Å. Both Fe(3)-O(1) bond lengths are 2.13 Å. Both Fe(3)-O(5) bond lengths are 2.08 Å. In the fourth Fe site, Fe(4) is bonded to one O(3), one O(8), two equivalent O(2), and two equivalent O(6) atoms to form FeO6 octahedra that share  a cornercorner with one Mg(1)O4 trigonal pyramid, edges with two equivalent Fe(4)O6 octahedra, edges with two equivalent Fe(2)O5 square pyramids, and edges with two equivalent Fe(1)O5 trigonal bipyramids. The Fe(4)-O(3) bond length is 2.09 Å. The Fe(4)-O(8) bond length is 2.06 Å. Both Fe(4)-O(2) bond lengths are 2.05 Å. Both Fe(4)-O(6) bond lengths are 2.05 Å. There are four inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(5) atom. The H(1)-O(5) bond length is 0.98 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(2) atom. The H(2)-O(2) bond length is 0.99 Å. In the third H site, H(3) is bonded in a single-bond geometry to one O(3) atom. The H(3)-O(3) bond length is 0.98 Å. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(4) atom. The H(4)-O(4) bond length is 0.98 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Fe(3) atoms. In the second O site, O(2) is bonded in a distorted single-bond geometry to two equivalent Fe(4) and one H(2) atom. In the third O site, O(3) is bonded in a distorted single-bond geometry to one Fe(4), two equivalent Fe(1), and one H(3) atom. In the fourth O site, O(4) is bonded in a distorted single-bond geometry to one Mg(1), two equivalent Fe(2), and one H(4) atom. In the fifth O site, O(5) is bonded in a distorted single-bond geometry to two equivalent Fe(3) and one H(1) atom. In the sixth O site, O(6) is bonded in a rectangular see-saw-like geometry to one Fe(1), one Fe(2), and two equivalent Fe(4) atoms. In the seventh O site, O(7) is bonded in a trigonal non-coplanar geometry to one Fe(3) and two equivalent Fe(1) atoms. In the eighth O site, O(8) is bonded to one Mg(1), one Fe(4), and two equivalent Fe(2) atoms to form distorted corner-sharing OMgFe3 tetrahedra.