Electronic Structure Control of Si-H Bond Activation by Transition Metals. 2. Valence Photoelectron Spectra of (η⁵-C₅H₄CH₃)Mn(CO)₂HSiPh₃, (η⁵-C₅H₄CH₃)Mn(CO)₂HSiHPh₂, and (η⁵-C₅H₄CH₃)Mn(CO)₂HSiFPh₂ (Ph = C₆H₅)

The He I photoelectron spectra of (η⁵-C₅H₄CH₃)Mn(CO)₂HSiHPh₂, (η⁵-C₅H₄CH₃)Mn(CO)₂HSiPh₃, and (η⁵-C₅H₄CH₃) Mn(CO)₂HSiFPh₂ (Ph = C₆H₅) have been obtained in order to measure the nature and extent of Si-H bond interaction with the transition-metal center in these complexes. The principal electronic structure factors contributing to the addition of the Si-H bond to the transition metal involve the interaction of the Si-H σand σ^* orbitals with the metal. The extent of Si-H σ^* interaction with the metal is obtained from the shape and splitting pattern of the metal-based ionization band. The electron distribution between the Si-H bond and the metal is indicated by the relative stabilities of the metal-based and ligand-based ionizations. It is found that the metal-based ionizations of these complexes reflect the formal d⁶ electron count at the metal center. Also, the small shifts of the valence ionizations reveal that the extent of electron charge density shift from the metal to the ligand is negligible. These observations show that the electronic structure of the Si-H interaction with the metal is in the initial stages of Si-H bond addition to the metal, before oxidative addition has become prevalent. The mechanism of interaction of the Si-H bond with the Mn center is predominantly through interaction of the filled Si-H -bonding orbital with empty metal orbitals. This contrasts with our previous photoelectron studies on (η⁵-C₅H₅)Mn(CO)₂HSiCl₃, where the experimental data showed this compound to be a nearly complete oxidative addition product with formation of discrete Mn-H and Mn-Si bonds. The combination of the present study with the previous results demonstrates that complexes can be obtained that stabilize different stages of activation of the Si-H bond. These stages range from weak coordination of the Si-H bond to the metal to complete oxidative addition, depending on the substituents on the silicon atom.