Er2Mo2C3 crystallizes in the monoclinic C2/m space group. Er(1) is bonded in a 5-coordinate geometry to two equivalent C(1) and three equivalent C(2) atoms. Mo(1) is bonded to one C(1) and three equivalent C(2) atoms to form a mixture of corner and edge-sharing MoC4 trigonal pyramids. There are two inequivalent C sites. In the first C site, C(1) is bonded to four equivalent Er(1) and two equivalent Mo(1) atoms to form CEr4Mo2 octahedra that share corners with twelve equivalent C(2)Er3Mo3 octahedra, edges with two equivalent C(1)Er4Mo2 octahedra, and faces with two equivalent C(2)Er3Mo3 octahedra. The corner-sharing octahedral tilt angles range from 43-57°. In the second C site, C(2) is bonded to three equivalent Er(1) and three equivalent Mo(1) atoms to form CEr3Mo3 octahedra that share corners with six equivalent C(1)Er4Mo2 octahedra, edges with six equivalent C(2)Er3Mo3 octahedra, and  a faceface with one C(1)Er4Mo2 octahedra. The corner-sharing octahedral tilt angles range from 43-57°.
Er2Mo2C3 crystallizes in the monoclinic C2/m space group. Er(1) is bonded in a 5-coordinate geometry to two equivalent C(1) and three equivalent C(2) atoms. Both Er(1)-C(1) bond lengths are 2.49 Å. There is one shorter (2.38 Å) and two longer (2.55 Å) Er(1)-C(2) bond lengths. Mo(1) is bonded to one C(1) and three equivalent C(2) atoms to form a mixture of corner and edge-sharing MoC4 trigonal pyramids. The Mo(1)-C(1) bond length is 2.02 Å. There are two shorter (2.10 Å) and one longer (2.11 Å) Mo(1)-C(2) bond length. There are two inequivalent C sites. In the first C site, C(1) is bonded to four equivalent Er(1) and two equivalent Mo(1) atoms to form CEr4Mo2 octahedra that share corners with twelve equivalent C(2)Er3Mo3 octahedra, edges with two equivalent C(1)Er4Mo2 octahedra, and faces with two equivalent C(2)Er3Mo3 octahedra. The corner-sharing octahedral tilt angles range from 43-57°. In the second C site, C(2) is bonded to three equivalent Er(1) and three equivalent Mo(1) atoms to form CEr3Mo3 octahedra that share corners with six equivalent C(1)Er4Mo2 octahedra, edges with six equivalent C(2)Er3Mo3 octahedra, and  a faceface with one C(1)Er4Mo2 octahedra. The corner-sharing octahedral tilt angles range from 43-57°.