MgNb2(RhO4)2 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form distorted MgO6 octahedra that share corners with two equivalent Rh(2)O6 octahedra,  a cornercorner with one Nb(1)O5 trigonal bipyramid, edges with two equivalent Rh(1)O6 octahedra,  an edgeedge with one Nb(1)O5 trigonal bipyramid, and  a faceface with one Rh(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), one O(5), and one O(6) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent Rh(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Rh(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-73°. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom. There are two inequivalent Rh sites. In the first Rh site, Rh(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted RhO6 octahedra that share corners with four equivalent Rh(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Nb(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 34-56°. In the second Rh site, Rh(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom to form distorted RhO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Rh(1)O6 octahedra, corners with three equivalent Nb(1)O5 trigonal bipyramids, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-56°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Nb(1), one Nb(2), and one Rh(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Rh(1), and one Rh(2) atom. In the third O site, O(3) is bonded in a distorted trigonal pyramidal geometry to one Mg(1), one Nb(1), one Nb(2), and one Rh(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Rh(1), and one Rh(2) atom. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(1), one Rh(1), and one Rh(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(2), and one Rh(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Nb(2), and one Rh(2) atom. In the eighth O site, O(8) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Nb(2), one Rh(1), and one Rh(2) atom.
MgNb2(RhO4)2 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), one O(7), and one O(8) atom to form distorted MgO6 octahedra that share corners with two equivalent Rh(2)O6 octahedra,  a cornercorner with one Nb(1)O5 trigonal bipyramid, edges with two equivalent Rh(1)O6 octahedra,  an edgeedge with one Nb(1)O5 trigonal bipyramid, and  a faceface with one Rh(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 52-55°. The Mg(1)-O(1) bond length is 2.34 Å. The Mg(1)-O(2) bond length is 2.05 Å. The Mg(1)-O(3) bond length is 2.02 Å. The Mg(1)-O(5) bond length is 2.18 Å. The Mg(1)-O(7) bond length is 2.03 Å. The Mg(1)-O(8) bond length is 2.12 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), one O(5), and one O(6) atom to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent Rh(2)O6 octahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with two equivalent Rh(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 41-73°. The Nb(1)-O(1) bond length is 1.88 Å. The Nb(1)-O(3) bond length is 2.07 Å. The Nb(1)-O(4) bond length is 1.93 Å. The Nb(1)-O(5) bond length is 1.98 Å. The Nb(1)-O(6) bond length is 2.09 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(6), one O(7), and one O(8) atom. The Nb(2)-O(1) bond length is 2.46 Å. The Nb(2)-O(2) bond length is 2.00 Å. The Nb(2)-O(3) bond length is 2.32 Å. The Nb(2)-O(6) bond length is 1.98 Å. The Nb(2)-O(7) bond length is 1.93 Å. The Nb(2)-O(8) bond length is 1.93 Å. There are two inequivalent Rh sites. In the first Rh site, Rh(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted RhO6 octahedra that share corners with four equivalent Rh(2)O6 octahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Nb(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 34-56°. The Rh(1)-O(2) bond length is 2.64 Å. The Rh(1)-O(3) bond length is 2.13 Å. The Rh(1)-O(4) bond length is 2.05 Å. The Rh(1)-O(5) bond length is 2.10 Å. The Rh(1)-O(6) bond length is 2.11 Å. The Rh(1)-O(8) bond length is 2.42 Å. In the second Rh site, Rh(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(7), and one O(8) atom to form distorted RhO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with four equivalent Rh(1)O6 octahedra, corners with three equivalent Nb(1)O5 trigonal bipyramids, and  a faceface with one Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 34-56°. The Rh(2)-O(1) bond length is 2.25 Å. The Rh(2)-O(2) bond length is 2.04 Å. The Rh(2)-O(4) bond length is 2.01 Å. The Rh(2)-O(5) bond length is 2.12 Å. The Rh(2)-O(7) bond length is 2.09 Å. The Rh(2)-O(8) bond length is 2.26 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 one Mg(1), one Nb(1), one Nb(2), and one Rh(2) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Mg(1), one Nb(2), one Rh(1), and one Rh(2) atom. In the third O site, O(3) is bonded in a distorted trigonal pyramidal geometry to one Mg(1), one Nb(1), one Nb(2), and one Rh(1) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(1), one Rh(1), and one Rh(2) atom. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(1), one Rh(1), and one Rh(2) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Nb(1), one Nb(2), and one Rh(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Nb(2), and one Rh(2) atom. In the eighth O site, O(8) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Nb(2), one Rh(1), and one Rh(2) atom.