The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH3)3 have been obtained in order to examine the bonding of silyl ligands to transition metals. The chemistry of both complexes has been studied previously in relation to models for catalytic intermediates in hydrosilation reactions. The nature of the metal-silicon σ bond and the possibility of π back-bonding from the metal to empty silicon d orbitals are of particular interest to the understanding of this chemistry. Previous observations of unusually short Fe-Si bond lengths in this class of complexes have led to the proposal that silane ligands are good π-acceptors. However, the splitting pattern of the “t2g-based” metal ionizations of (η5-C5H5)-Fe(CO)2Si(CH3)3 shows no evidence of stabilization of metal d orbitals as would occur with π-back-bonding to the silane. The splitting is essentially the same as observed in the spectrum of (η5-C5H5)Fe(CO)2H, where the hydride clearly has no π-acceptor capability. This same splitting is also observed in the spectrum of (η5-C5H5)Fe(CO)2CH3. A reduced splitting is observed in the spectrum of (η5-C5H5)Fe(CO)2SiCl3. Evaluation of these ionizations shows that SiCl3 is a better π-acceptor than CN and is about half as effective as CO at π stabilization of the metal ionizations. The π-back-bonding to SiCl3 probably involves significant portions of the S-Cl σ* orbitals. The short Fe-Si bond in the SiCl3 complex is a combination of π-back-bonding and ionic bonding contributions. The ionizations of (η5-C5H5)Fe(CO)2SiCl3 are also compared with the ionizations of the “isoelectronic” (η5-C5H5)Mn(CO)2-HSiCl3 complex. The manganese complex is related to the iron complex by a “hydride shift” from the iron nucleus, and the ionization shifts help to reveal the extent of oxidative addition of the Si-H bond to the manganese center. The strength of the Mn-SiCl3 bond contributes to the nearly complete oxidative addition of the Si-H bond to the metal. The implications of these results to the proposed mechanisms of hydrosilation reactions are discussed. The ionization trends in these complexes favor olefin insertion into the metal-silyl bond in the case of trialkylsilyl complexes.
ASJC Scopus subject areas
- Colloid and Surface Chemistry