Synthesis and characterization of molybdenum carbide clusters Mo _nC_4n (n=1 to 4)

Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)₆ vapor produced uftrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry Mo_nC_4n (n=1 to 4). The Mo_nC_4n^- (n=2 to 4) ions are completely unreactive with NH₃, H₂O, and O₂, suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC₄ units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers.