Valence Ionizations of Olefins Coordinated to Metals. Olefin Dicarbonyl(η⁵-(methyl and pentamethyl)cyclopentadienyl)manganese Complexes

The He I and He II valence photoelectron spectra of (η⁵-C₅H_5-n(CH₃)_n)Mn(CO)₂L (n = 1 and 5; L = C₂H₄ and C₃H₆) are presented. The synthesis of the pentamethylcyclopentadienyl metal ethylene complex is reported for the first time. This complex is particularly helpful in revealing the ionization characteristics of the metal-olefin complexes. In each case the ionization that is associated primarily with the olefin π bond (2e⁻ donor to the metal) is shifted to lower binding energy in the complex compared to the binding energy of the free ligand. Molecular orbital calculations are reported that agree very well with the shifts in ionization energies. The relative magnitudes of individual metal-olefin orbital interactions are evaluated. It is found that the ionizations and stability of the complexes are sensitive to the geometry changes that accompany coordination of the olefin. These distortions are associated with a lowered carbon-carbon bond strength and an increased metal-olefin bond strength through increased π-donor/π*-acceptor interactions.