The crystal and molecular structure of tris-[cis-1,2-di(trifluoromethyl)ethylene-1,2-diselenato]molybdenum, Mo[Se₂C₂(CF₃)₂]₃, has been determined from three-dimensional single-crystal X-ray data, collected by counter methods, and refined by least-squares methods to R 0·041 for 457 independent nonzero reflections. The neutral complex, which has C_3h symmetry, crystallizes in space group P6₃/m of the hexagonal system with Z= 2 in a cell of dimensions a= 10·03 (2) and c= 14·30 (2)Å. The six selenium atoms surround the molybdenum atom in a trigonal-prismatic co-ordination with a Mo–Se distance of 2·492(2)Å. The relatively short interligand Se ⋯ Se distance of 3·222(3)Å, as compared with an intraligand value of 3·317(5)Å, is believed to be indicative of interdonor atom bonding interactions which have also been postulated for the neutral tris-dithiolene complexes as a means of stabilizing the unusual trigonal-prismatic co-ordination geometry.