Crystal structure of [1,2-bis(dimethylarsino)trifluoroethane]tetracarbonylmolybdenum, (Me2AsCHF·CF2AsMe2)Mo(CO)4

Crystals of [1,2-bis(dimethylarsino)trifluoroethane]tetracarbonylmolybdenum, (Me2AsCHF·CF2AsMe2)Mo(CO)4, are orthorhombic, a= 18·26, b= 14·87, c= 12·52 A, Z= 8, space group Pbca. The structure was determined with Mo-Kα diffractometer data by Patterson, electron-density, and full-matrix least-squares methods, the final R being 0·052 for 1757 observed reflexions. The molybdenum atom has a slightly distorted octahedral co-ordination, with As–Mo–As angle 81·9(1)° and Mo–As bond distances 2·563(2) and 2·589(2)A. The five-membered ring is non-planar, with As–C–C–As dihedral angle 22°, and the hydrogen atom in an equatorial position. The C–C bond distance is extremely short, 1·30(2)A, and the axial C–F bond lengths, 1·72(3) and 1·73(2)A, are very significantly longer than the equatorial C–F bond, 1·38(2)A, and than a normal C–F bond, 1·33 A. The C–C–F-(axial) angles are very far from tetrahedral (85 and 82°). These unusual features can be explained by a contribution from an ionic resonance structure, [(Me2AsCH:CFAsMe2)Mo(CO)4]2+2F–.