Gas-Phase Helium I Photoelectron Spectra of Methinyltricobalt Enneacarbonyl Clusters

The electronic structure of Co₃(CO)₉CY clusters, where Y = H, F, Cl, Br, I, CH₃, CF₃, or COOCH₃, is discussed on the basis of their He I excited gas-phase photoelectron spectra. A qualitative energy level diagram has been afforded which allows a satisfactory discussion of the PE data. The most interesting feature of our interpretative model is the presence of a molecular orbital of e symmetry which accounts for a large portion of the interaction between the Co₃ triangle and the apical carbon atom C_ap and for the electron transmission from the apical substituent Y and the cobalt atoms via a resonance-type mechanism. In agreement with previous NQR and IR data, evidence for significant Y→C_ap π-bonding donation with Y = Cl, Br, or I is presented. The relationships existing between the present clusters and molecules of type M₃(CO)₁₂ and (µ-CH₂)[ML_n]₂ have been stressed.