COs is Molybdenum Carbide MAX Phase-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Os2- is bonded to six equivalent C2+ atoms to form a mixture of edge, face, and corner-sharing OsC6 octahedra. The corner-sharing octahedral tilt angles are 46°. All Os–C bond lengths are 2.18 Å. C2+ is bonded to six equivalent Os2- atoms to form a mixture of distorted edge and corner-sharing COs6 pentagonal pyramids.