Electronic structure of transition metal-silicon bonds. Valence photoelectron spectra of (η5-C5H5)Fe(CO)2L complexes (L = SiCl3, Si(CH3)3)

Dennis L Lichtenberger, Anjana Rai-Chaudhuri

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Abstract

The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH 3)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(CH 3)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)2SiCl 3. 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 Si-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-HSiCl 3 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.

Original languageEnglish (US)
Pages (from-to)2923-2930
Number of pages8
JournalJournal of the American Chemical Society
Volume113
Issue number8
StatePublished - Apr 10 1991

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Silicon
Carbon Monoxide
Photoelectrons
Ionization
Electronic structure
Transition metals
Metals
Silanes
Hydrides
Manganese
Stabilization
Ligands
Iron
Bond length
iron pentacarbonyl
Alkenes
Olefins

ASJC Scopus subject areas

  • Chemistry(all)

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@article{c651973b3e5341aab08be4df7f7db947,
title = "Electronic structure of transition metal-silicon bonds. Valence photoelectron spectra of (η5-C5H5)Fe(CO)2L complexes (L = SiCl3, Si(CH3)3)",
abstract = "The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH 3)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(CH 3)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)2SiCl 3. 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 Si-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-HSiCl 3 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.",
author = "Lichtenberger, {Dennis L} and Anjana Rai-Chaudhuri",
year = "1991",
month = "4",
day = "10",
language = "English (US)",
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pages = "2923--2930",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
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T1 - Electronic structure of transition metal-silicon bonds. Valence photoelectron spectra of (η5-C5H5)Fe(CO)2L complexes (L = SiCl3, Si(CH3)3)

AU - Lichtenberger, Dennis L

AU - Rai-Chaudhuri, Anjana

PY - 1991/4/10

Y1 - 1991/4/10

N2 - The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH 3)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(CH 3)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)2SiCl 3. 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 Si-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-HSiCl 3 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.

AB - The HeI and HeII photoelectron spectra of (η5-C5H5)Fe(CO)2SiCl3 and (η5-C5H5)Fe(CO)2Si(CH 3)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(CH 3)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)2SiCl 3. 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 Si-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-HSiCl 3 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.

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