Electronic Structure Factors of Si—H Bond Activation by Transition Metals. The Valence Photoelectron Spectrum of (η⁵-C₅H₅)Mn(CO)₂HSiCl₃

The valence photoelectron spectra of HSiCl₃ and CpMn(CO)₂HSiCl₃ (Cp = η⁵-C₅H₅₎ have been obtained to nrovide a measure of the electronic structure factors that contribute to the interaction of the Si—H bond with a transition-metal center. The lowest valence ionizations of other CpMn(CO)₂(ligand) complexes are metal-based and reflect the formal d⁶ electron count at the metal. The lowest valence ionizations in the He I photoelectron spectrum of CpMn(CO)₂HSiCl₃ are more stable and split over a wider energy range than has been observed previously. The chlorine lone pair based ionizations of HSiCl₃ coordinated to the metal have shifted about 1 eV to lower ionization energy from those of the free HSiCl₃ molecule. Both the stabilization of the metal-based ionizations and the destabilization of the ligand-based ionizations show that electron charge shift from the metal to the ligand more than compensates for the initial σDonation from the Si—H bond to the metal in CpMn(CO)₂HSiCl₃, as expected if the bonding has proceeded significantly toward oxidative addition. The relative intensity of the higher ionization energy component in the metal ionization region decreases with He II excitation, showing that this band is actually associated more with ligand character. This He I/He II relative intensity behavior corresponds to a formal d⁴ electron count for the metal, consistent with the Mn(III) oxidation state. Fenske-Hall calculations indicate that the metal d hybrid orbitals available for the Mn-Si and Mn-H bonds form an acute angle between 50° and 70°, and these directed hybrids are responsible for the close proximity of the silicon and hydrogen atoms in the complex. These results are related to the other physical and chemical properties of this complex and are contrasted with the results of our similar photoelectron studies on the interaction of a carbon—hydrogen bond with the metal center in (η³-C₆H₉)Mn(CO)₃, where the interaction primarily involves donation of C-H σ bonding orbital electron density into the empty metal orbitals in the formation of a 3-center 2-electron bond.