Negative Ion Photoelectron Spectroscopy of Coordinatively Unsaturated Group VI Metal Carbonyls: Cr(CO)₃, Mo(CO)₃, and W(CO)₃

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-nm spectra were obtained at an electron kinetic energy resolution of 5 meV using a new apparatus with improved mass resolution, which is described in this report. 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. Principal force constants estimated from the measured vibrational frequencies of the neutral molecules are consistent with stronger metal-ligand bonding in the coordinatively unsaturated tricarbonyls than in the corresponding hexacarbonyl complexes. 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. Electron affinities of 1.349 ± 0.006 eV for Cr(CO)₃, 1.337 ± 0.006 eV for Mo(CO)₃, and 1.859 ± 0.006 eV for W(CO)₃ are obtained. 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. Related studies of the atomic anions yield improved values for the electron affinities of Cr (0.675 ± 0.004 eV), Mo (0.747 ± 0.004 eV), and W (0.817 ± 0.004 eV).