The Effects of Methyl Group Substitution on Metal-Coordinated Cyclopentadienyl Rings. The Core and Valence Ionizations of Methylated Tricarbonyl(η⁵-cyclopentadienyl)metal Complexes

Gas-phase He I, He II, and Mg Kα photoelectron spectra are reported for molecules of the type (η⁵-C₅H_5-n⁻ (CH₃)_n)M(CO)₃ where = 0, 1, 5 and M = Mn, Re. The influence of methyl groups on the cyclopentadienyl ring is monitored by shifts in both core and valence ionization energies. This enables effective separation of electron density transfer (inductive) and ring-methyl orbital overlap (hyperconjugative) effects. While the shift in the ring e₁“ ionization is found to be primarily a hyperconjugative effect, the shift in the metal valence ionizations is caused essentially entirely by a shift of electron density toward the metal atom. A greater proportion of this increased density is transferred to the carbonyls in the rhenium complexes than in the manganese complexes, indicating the greater back-bonding ability of the third-row atom. Further evidence of extensive Re-CO back-bonding is provided by the presence of vibrational fine structure on one of the predominantly metal ionizations of the rhenium complexes. This structure is the vibrational progression of the symmetric metal-carbon(CO) stretching mode. The long vibrational progression observed in this band and the frequency of the M-C stretch in the positive ion are direct evidence of considerable tr back-bonding from the metal to the carbonyls. The observed vibrational structure in the spin-orbit split rhenium d ionizations also leads to a definitive interpretation of the pattern of metal ionizations in such complexes. The origin of the characteristic splitting of the predominantly ring e₁“ ionization is also considered in detail. The data suggest that the carbon-carbon bond distances in the ring are distorted an average of 0.01 to 0.02 Å from fivefold symmetry when coordinated to a d⁶ ML₃ species. This is the first indication from gas-phase spectroscopy for such distortions.