Electronic Structure Factors of Si–H Bond Activation by Transition Metals. Valence Photoelectron Spectra of (η⁵-C₅H₄CH₃)Mn(CO)(PMe₃)HSiCl₃ and (η⁵-C₅H₄CH₃)]Mn(CO)(PMe₃)HSiHPh² (Me = CH³, Ph = C₆H₅)

The valence photoelectron spectra of (η⁵-C₅H₄CH₃)Mn(CO)(L)HSiCl₃ and (η⁵-C₅H₄CH₃)Mn(CO)(L)HSiHPh₂, where L is CO or P(CH₃)₃, are compared to determine the effect of ligand substitution at the metal center on Si–H bond activation. Metal centers that are more electron rich may promote more complete oxidative addition of the Si–H bond to the metal. The shifts in the metal and ligand ionization energies and the relative intensities of ionizations in the He I and He II photoelectron experiments show that the metal in (η⁵-C₅H₄CH₃)Mn(CO)(PMe₃)HSiCl₃ is best represented by a formal oxidation state of III (d⁴ electron count). This indicates nearly complete oxidative addition of the Si–H bond to the metal center and results in independent Mn–H and Mn–Si bonds. In contrast, the splitting and intensity pattern of the metal-based ionizations of (η⁵-C₅H₄CH₃)Mn(CO)(PMe₃)-HSiHPh₂ reflect the formal d⁶ electron count of a metal corresponding to oxidation state I. The extent of electron charge density shift from the metal to the ligand is also small, as evidenced by the negligible shifts of these ionizations from those of the related (η⁵-C₅H₄CH₃)Mn(CO)₂(PMe₃) complex. These observations indicate that the electronic structure of the Si–H interaction with the metal in this complex is in the initial stages of Si–H bond addition to the metal, before oxidative addition has become prevalent. Comparison with the previously reported photoelectron spectra of (η⁵-C₅H₅)Mn(CO)₂HSiCl₃ and (η⁵-C₅H₄CH₃)Mn(CO)₂HSiHPh₂ shows that the Si–H bond interaction with the transition metal is affected more by alkyl and halogen substitutions on silicon than by substitution of a carbonyl with typical two-electron donor ligands at the metal center.