Gas-phase study of the group VI transition metal tricarbonyl complexes by negative-ion photoelectron spectroscopy (Poster Paper)

Photoelectron spectra are reported for Cr(CO)₃^-, Mo(CO)₃^- and W(CO)₃^- anions prepared from the corresponding metal hexacarbonyls in a flowing afterglow ion source. The 488.0 nm spectra were obtained at an electron kinetic energy resolution of 5 meV using a newly constructed apparatus. The spectra exhibit transitions between the ground electronic states of the anions and the neutral molecules, and they show weak activity in the symmetric CO stretching, MC stretching, MCO bending and CMC bending vibrational modes. The observed vibrational structure indicates that the anions, like the neutral molecules, have C_3v equilibrium geometries. Force constants estimated for the neutral M(CO)₃ molecules from the fundamental vibrational frequencies measured here are consistent with stronger metal-ligand bonding in the coordinatively unsaturated complexes than in the corresponding hexacarbonyls. Franck-Condon analyses of the spectra indicate only small differences between the equilibrium bond lengths and bond angles of the anions and the corresponding neutral molecules. The electron affinity pattern observed among the three group VI metal tricarbonyls is compared with characteristic trends within triads of transition metal atoms, and within the coinage metal dimer series. This comparison, combined with the results of previously reported theoretical calculations, suggests that the extra electron in the M(CO)₃^- anions occupies an sp hybrid orbital. Electron affinities of 1.349 eV, 1.337 eV, and 1.859 eV (all ± 0.006 eV) are obtained for Cr(CO)₃, Mo(CO)₃, and W(CO)₃ respectively.