Comparative mass spectrometry of the group 6B hexacarbonyls and pentacarbonyl thiocarbonyls

Mass spectra, ionization energies, fragmentation energies, ionic and neutral dissociation energies, and heats of formation for the title compounds are measured and compared. Substitution of CS for CO in going from M(CO)6 to M(CO)5CS results in (a) a slight modification of the mass spectra, if only the M(CO)/sub n/CS+ and M+ ions are considered, (b) a lowering of the ionization energies by 0.1 to 0.4 eV, (c) a reduction of approximately 0.1 eV in the average M-CO bond energies in both the molecule and the molecular ion, and (d) a reduction of 3.0 +- 0.3 eV in the heats of formation of the molecules. Within each molecule or molecular ion the M-CS bond is stronger than the M-CO bonds by 1.5 to 2.0 eV. Both the M-CS and the M-CO bonds increase in strength in going from Cr to Mo to W. Comparison of calorimetric and mass spectrometric data for M(CO)6 indicates that each neutral CO fragment leaves the ionization site in its first vibrationally excited state. 10 tables