THE CO-CO INTERACTIONS IN TRANSITION-METAL HEXACARBONYLS STUDIED BY PENNING IONIZATION ELECTRON-SPECTROSCOPY - CR(CO)6, MO(CO)6, AND W(CO)6

Penning ionization electron spectra resulting from thermal collisions of He*(2 3S) metastable atoms with gaseous Cr(CO)6, Mo(CO)6, and W(CO)6 were measured to probe the CO-CO interactions. The energy splitting of the CO 5-sigma-derived levels [I.P.(8a1g) - I.P.(8t1u)] was determined for the first time. Further, the spectra show that the through-space interaction among the CO 5-sigma orbitals not only stabilizes the orbital energy of the CO 5-sigma-derived 8a1g molecular orbital, but also gives rise to a diffuse electron distribution. The CO-CO interactions in the transition-metal (TM) carbonyls were compared to those in the CO overlayer formed on TM surfaces.