Synthesis and Characterization of Molybdenum Carbide Clusters Mo$_n$C$_{4n}$ (n = 1 to 4)

Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)$_6$ vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry Mo$_n$C$_{4n}$ (n = 1 to 4). The Mo$_n$C$_{4n}^-$ (n = 2 to 4) ions are completely unreactive with NH$_3$, H$_2$O, and O$_2$, 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 MoC$_4$ 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.