The first observations of metal-carbon vibrational structure in photoionization bands are reported. Attention is focused on the predominantly metal d ionizations of M(CO)6 (M = Cr, Mo, and W), and the methods for obtaining high resolution and very high signal-to-noise He I ionization data are detailed. The 2T2g ionization band of Cr(CO)6 and the spin-orbit split 2E″ and 2U′ bands of W(CO)6 show distinct vibrational progressions which correspond to the totally symmetric (a1g) metal-carbon stretching mode in the positive ion states. The metal-carbon stretching frequencies are found to be significantly less in the positive ion states than in the ground states, indicating a reduction of metal-carbon bond order upon the loss of a t2g electron. Evaluation of the vibrational progressions shows that the metal-carbon bond length increases on the order of 0.10 Å upon t2g ionization in the case of W(CO)6 and about 0.14 Å in the case of Cr(CO)6. In addition, the beginning of a short progression in the a1g carbon-oxygen stretching mode is observed in the Mo(CO)6 spectrum and is clearly seen in the W(CO)6 spectrum. All of these observations show that removal of an electron from the predominantly metal t2g orbitals, which are strictly π symmetry with respect to the carbonyls, substantially weakens the metal-to-carbonyl bond.
ASJC Scopus subject areas
- Colloid and Surface Chemistry