Core and valence-shell electronic excitation of the group VI transition-metal hexacarbonyls Cr(CO)6, Mo(CO)6, and W(CO)6 by high-resolution electron energy loss spectroscopy

Abstract

Core (inner-shell) and valence-shell electron energy loss spectra of the group VIA hexacarbonyls Cr(CO)6, Mo(CO)6, and W(CO)6 are compared with those of free CO and Ni(CO)4 under kinematic conditions where the spectra are dominated by electric dipole transitions. The transition-metal carbonyl inner-shell C 1s and O 1s spectra show some major similarities to those of free CO, and in particular they exhibit intense 1s → π* and 1s → σ* transitions. For the C 1s →* transitions vibrational structure was resolved. This vibrational structure is influenced by metal d →* CO π* back-bonding effects. However, some significant differences also exist between the spectra of the carbonyl complexes and those of free CO, principally due to greater orbital relaxation accompanying creation of a core hole in the transition-metal compounds and the larger number of final states accessible. Tentative assignments are suggested for the hexacarbonyl spectra with the use of molecular orbital energy level and term value considerations. The significance of the results with regard to the strength and nature of metal-CO bonding in transition-metal carbonyl complexes is discussed, and the potential usefulness of inner-shell electron energy loss spectroscopy as a tool for investigating the bonding in transition-metal species in general is assessed.