13C NMR Chemical Shielding Tensor of the Bridging Methylene Unit in cis-(μ-CH2)(μ-CO)[FeCp(CO)]2

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The principal elements of the 13C NMR chemical shielding tensor have been determined for the bridging methylene unit in cis-(μ-CH2)(μ-CO)[FeCp(CO)]2 from a combination of a Herzfeld-Berger analysis of the CP/MAS spectrum and a nonlinear least-squares fit of the proton-decoupled 13C powder pattern. The 13CH2 unit is both spatially isolated from other magnetic nuclei in the solid and largely motionally decoupled from dipolar interactions with the Cp ring protons. For the purpose of interpreting the proton-coupled 13C powder pattern, we have assumed that the carbon site of the dimetallocyclopropane unit lies on the intersection of two perpendicular mirror planes of symmetry. With this assumption, there are six possible relative orientations of the 13CH2 unit with respect to the principal axis system of the 13C chemical shielding tensor; simulations of the proton-coupled 13C powder patterns have been compared to the experimental spectrum, and the orientation of the chemical shielding tensor with respect to the molecular frameworks has been assigned. The C-H bond length and the H-C-H bond angle were obtained from the dipolar coupling tensor. A single, very large paramagnetic chemical shielding tensor element is a result of relatively weak carbon-metal bonds through two carbon atomic p orbitals and strong C-H bonds with the remaining carbon p orbital. Thus, these results are consistent with a molecular orbital analysis in which the methylene unit has σ-donating a1 and π-accepting b1 valence orbitals. © 1991, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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