Nuclear magnetic resonance spectroscopy of metal cyclopentadienyls V. Metallotropic rearrangement in silicon cyclopentadienyls: 1,2 shift and spectral regularities in σ-cyclopentadienyls

  • Sergeyev N
  • Avramenko G
  • Kisin A
 et al. 
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The saturation transfer technique, INDOR, and related computations have been used to provide a complete analysis of the PMR spectra of isomeric (trimethylsilyl)cyclopentadienes whilst the spectra of C5H5Si(CH3)2Cl and C5H5SiCl3have been partially analysed. The main vinylic isomer in the series C5H5Si(CH3)nCl3-n(n is 0, 1, 2, 3) is the 1-substituted species while the 5-substituted isomers are subject to a metallotropic rearrangement. From the assumption that3J(HH) > 0 and4J(HH) < 0, it follows that the upfield olefinic signal of the 5-substituted isomers is related to the 1,4 protons. Unsymmetrical broadening shows that in all the compounds the migration proceeds predominantly through the 1,2 shift with a free energy of activation of 15 to 16 kcal/mole. As the electronegativity of the substituents attached to the silicon atom increases, the equilibrium associated with the prototropic rearrangement is shifted towards formation of the vinylic isomers while the corresponding metal migration rate decreases. A thorough analysis of the coupling constants, J(HH), and chemical shifts, δ(H), associated with the protons of σ-cyclopentadienyl compounds shows that the constant3J15is always positive and equal to 1.2±0.2 Hz while4J25is, as a rule, negative and variable within the range -1.6 to +0.2 Hz. In the absence of electron-withdrawing groups (e.g. as in carbonyls) at the metal, the condition δ14< δ23invariably applies. © 1971.

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  • N. M. Sergeyev

  • G. I. Avramenko

  • A. V. Kisin

  • V. A. Korenevsky

  • Yu A. Ustynyuk

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