Rydberg structures in the gas-phase electronic absorptionspectra of η-cycloheptatrienyl η-cyclopentadienyl complexes ofmolybdenum

The gas-phase electronic absorption spectra of (Ch)(Cp)Mo and (Ch)(Cp′)Mo (Ch=η ⁷ -C ₇ H ₇ , Cp=η ⁵ -C ₅ H ₅ , Cp′=η ⁵ -C ₅ H ₄ CH ₃ ) have been measured for the first time and compared with those recorded in n-heptane solution. The gas-phase spectra reveal rich Rydberg structures which disappear on going to the solution phase. The Rydberg bands correspond to transitions from the non-bonding molybdenum 4d _z² orbital to Rns, Rnp and Rnd levels. On the basis of analysis of Rydberg frequencies, the first ionization potentials of (Ch)(Cp)Mo and (Ch)(Cp′)Mo have been determined as 45890 cm ^-1 (5.69 eV) and 45300 cm ^-1 (5.62 eV), respectively. Assignments based on term values and effective quantum numbers of the Rydberg transitions have been made. The R5s, R5p and R5d bands reveal vibronic components assigned to totally symmetric CC stretches, CH umbrella vibrations and metal–ring skeletal modes. Methylation of (Ch)(Cp)Mo results in splitting of the R5p _x,y and R5d _xz,yz transitions and appearance of the R5d _xy and R5d _{x² -y²} excitations. These changes are in agreement with a molecular symmetry reduction from C _∞v to C _s . The spectrum of (Ch)(Cp′)Mo gives the first example of an interpretable structure of the lowest Rnp and Rnd transitions in a methylated sandwich complex. The differences between the Rydberg structures of the (Ch)(Cp)Mo and (Ch)(Cp′)Mo spectra in the n>5 region are due to changes in Rydberg–valence interactions and relative band intensities on methylation.