RYDBERG STRUCTURES IN THE GAS-PHASE ELECTRONIC ABSORPTION-SPECTRA OF ETA-CYCLOHEPTATRIENYL ETA-CYCLOPENTADIENYL COMPLEXES OF MOLYBDENUM

The gas-phase electronic absorption spectra of (Ch)(Cp)Mo and (Ch)(Cp' )Mo (Ch = eta(7)-C7H7, Cp = eta(5)-C5H5, Cp' = eta(5)-C5H4CH3) have be en measured for the first time and compared with those recorded in n-h eptane solution. The gas-phase spectra reveal rich Rydberg structures which disappear on going to the solution phase. The Rydberg bands corr espond to transitions from the non-bonding molybdenum 4d(z2) orbital t o Rns, Rnp and Rnd levels. On the basis of analysis of Rydberg frequen cies, the first ionization potentials of (Ch)(Cp)Mo and (Ch)(Cp')Mo ha ve been determined as 45 890 cm(-1) (5.69 eV) and 45 300 cm(-1) (5.62 eV), respectively. Assignments based on term values and effective quan tum numbers of the Rydberg transitions have been made. The R5s, R5p an d R5d bands reveal vibronic components assigned to totally symmetric C C stretches, CH umbrella vibrations and metal-ring skeletal modes. Met hylation of (Ch)(Cp)Mo results in splitting of the R5p(x, y) and R5d(x x, yz) transitions and appearance of the R5d(xy) and R5d(x2-y2) excita tions. These changes are in agreement with a molecular symmetry reduct ion from C-infinity v to C-s. The spectrum of (Ch)(Cp')Mo gives the fi rst example of an interpretable structure of the lowest Rnp and Rnd tr ansitions in a methylated sandwich complex. The differences between th e 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 re lative band intensities on methylation.