Mg(ScSe2)2 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(3), two equivalent Se(4), and three equivalent Se(1) atoms to form distorted MgSe7 pentagonal bipyramids that share corners with four equivalent Sc(1)Se6 octahedra, corners with four equivalent Sc(2)Se6 octahedra,  an edgeedge with one Sc(1)Se6 octahedra, edges with four equivalent Sc(2)Se6 octahedra, edges with two equivalent Mg(1)Se7 pentagonal bipyramids, and faces with two equivalent Mg(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 12-64°. There are two inequivalent Sc sites. In the first Sc site, Sc(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form ScSe6 octahedra that share corners with three equivalent Sc(2)Se6 octahedra, corners with four equivalent Mg(1)Se7 pentagonal bipyramids, edges with two equivalent Sc(2)Se6 octahedra, edges with four equivalent Sc(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 51-59°. In the second Sc site, Sc(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form ScSe6 octahedra that share corners with three equivalent Sc(1)Se6 octahedra, corners with four equivalent Mg(1)Se7 pentagonal bipyramids, edges with two equivalent Sc(1)Se6 octahedra, edges with two equivalent Sc(2)Se6 octahedra, and edges with four equivalent Mg(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 51-59°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Sc(2) atoms to form SeMg3Sc2 square pyramids that share corners with two equivalent Se(4)Mg2Sc3 square pyramids, corners with six equivalent Se(3)Mg2Sc3 trigonal bipyramids, edges with three equivalent Se(4)Mg2Sc3 square pyramids, edges with four equivalent Se(1)Mg3Sc2 square pyramids, and edges with two equivalent Se(3)Mg2Sc3 trigonal bipyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Sc(2) and three equivalent Sc(1) atoms. In the third Se site, Se(3) is bonded to two equivalent Mg(1), one Sc(1), and two equivalent Sc(2) atoms to form distorted SeMg2Sc3 trigonal bipyramids that share corners with two equivalent Se(4)Mg2Sc3 square pyramids, corners with six equivalent Se(1)Mg3Sc2 square pyramids, edges with two equivalent Se(1)Mg3Sc2 square pyramids, edges with three equivalent Se(4)Mg2Sc3 square pyramids, and edges with two equivalent Se(3)Mg2Sc3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Sc(2), and two equivalent Sc(1) atoms to form SeMg2Sc3 square pyramids that share corners with two equivalent Se(1)Mg3Sc2 square pyramids, corners with two equivalent Se(3)Mg2Sc3 trigonal bipyramids, edges with two equivalent Se(4)Mg2Sc3 square pyramids, edges with three equivalent Se(1)Mg3Sc2 square pyramids, and edges with three equivalent Se(3)Mg2Sc3 trigonal bipyramids.
Mg(ScSe2)2 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(3), two equivalent Se(4), and three equivalent Se(1) atoms to form distorted MgSe7 pentagonal bipyramids that share corners with four equivalent Sc(1)Se6 octahedra, corners with four equivalent Sc(2)Se6 octahedra,  an edgeedge with one Sc(1)Se6 octahedra, edges with four equivalent Sc(2)Se6 octahedra, edges with two equivalent Mg(1)Se7 pentagonal bipyramids, and faces with two equivalent Mg(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 12-64°. Both Mg(1)-Se(3) bond lengths are 3.09 Å. Both Mg(1)-Se(4) bond lengths are 2.81 Å. There are two shorter (2.76 Å) and one longer (2.81 Å) Mg(1)-Se(1) bond length. There are two inequivalent Sc sites. In the first Sc site, Sc(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form ScSe6 octahedra that share corners with three equivalent Sc(2)Se6 octahedra, corners with four equivalent Mg(1)Se7 pentagonal bipyramids, edges with two equivalent Sc(2)Se6 octahedra, edges with four equivalent Sc(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 51-59°. The Sc(1)-Se(3) bond length is 2.74 Å. Both Sc(1)-Se(4) bond lengths are 2.70 Å. There are two shorter (2.68 Å) and one longer (2.82 Å) Sc(1)-Se(2) bond length. In the second Sc site, Sc(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form ScSe6 octahedra that share corners with three equivalent Sc(1)Se6 octahedra, corners with four equivalent Mg(1)Se7 pentagonal bipyramids, edges with two equivalent Sc(1)Se6 octahedra, edges with two equivalent Sc(2)Se6 octahedra, and edges with four equivalent Mg(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 51-59°. The Sc(2)-Se(2) bond length is 2.75 Å. The Sc(2)-Se(4) bond length is 2.74 Å. Both Sc(2)-Se(1) bond lengths are 2.72 Å. Both Sc(2)-Se(3) bond lengths are 2.69 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Sc(2) atoms to form SeMg3Sc2 square pyramids that share corners with two equivalent Se(4)Mg2Sc3 square pyramids, corners with six equivalent Se(3)Mg2Sc3 trigonal bipyramids, edges with three equivalent Se(4)Mg2Sc3 square pyramids, edges with four equivalent Se(1)Mg3Sc2 square pyramids, and edges with two equivalent Se(3)Mg2Sc3 trigonal bipyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Sc(2) and three equivalent Sc(1) atoms. In the third Se site, Se(3) is bonded to two equivalent Mg(1), one Sc(1), and two equivalent Sc(2) atoms to form distorted SeMg2Sc3 trigonal bipyramids that share corners with two equivalent Se(4)Mg2Sc3 square pyramids, corners with six equivalent Se(1)Mg3Sc2 square pyramids, edges with two equivalent Se(1)Mg3Sc2 square pyramids, edges with three equivalent Se(4)Mg2Sc3 square pyramids, and edges with two equivalent Se(3)Mg2Sc3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Sc(2), and two equivalent Sc(1) atoms to form SeMg2Sc3 square pyramids that share corners with two equivalent Se(1)Mg3Sc2 square pyramids, corners with two equivalent Se(3)Mg2Sc3 trigonal bipyramids, edges with two equivalent Se(4)Mg2Sc3 square pyramids, edges with three equivalent Se(1)Mg3Sc2 square pyramids, and edges with three equivalent Se(3)Mg2Sc3 trigonal bipyramids.