Gas-phase electronic absorption spectra of metallocene dihydrides M(η5-C5H5)2H2 (M=Mo, W)

The gas-phase electronic absorption spectrum of MoCp2H2 (Cp=η5-C5H5) reveals an intense peak at 35 500 cm−1 which is absent in the spectrum measured in n-pentane solution. The spectrum of WCp2H2 shows a shoulder at 35 000 cm−1 which disappears on going from the gas to the condensed phase. The term values of these features (16 100 and 16 600 cm−1, respectively) are appropriate for their assignment as the transitions from the HOMO to the lowest Rydberg p-orbitals. The gas-phase spectra of MoCp2H2 and WCp2H2 give the first examples of clearly defined Rydberg transitions in the organometallic molecules with σ-bonded ligands. Appearance of Rydberg bands confirms that the HOMO of these complexes is really non-bonding.