Trigonal Prismatic Structure of Tris(butadiene)molybdenum and Related Complexes Revisited: Diolefin or Metallacyclopentene Coordination?

Kaupp, Martin, Kopf, Thomas, Murso, Alexander, Stalke, Dietmar, Strohmann, Carsten, Hanks, John R, Cloke, F Geoffrey N and Hitchcock, Peter B (2002) Trigonal Prismatic Structure of Tris(butadiene)molybdenum and Related Complexes Revisited: Diolefin or Metallacyclopentene Coordination? Organometallics, 21 (23). pp. 5021-5028. ISSN 02767333

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The unusual trigonal prismatic structure of tris(butadiene)molybdenum, reported in 1975 by Skell, has been revisited by extensive quantum chemical calculations and by a low-temperature single-crystal X-ray diffraction study. While a trigonal prismatic coordination arrangement is confirmed by DFT and MP2 structure optimizations, the calculations provide very different bond lengths than earlier crystallographic studies: Due to appreciable back-bonding, the terminal Mo-C1 bonds are significantly shorter than the central Mo-C2 bonds (ca. 2.29 vs ca. 2.36 Å), and the central (C2-C2A) bonds are actually shorter than the terminal ones (ca. 1.40 vs ca. 1.44 Å), as found previously for substituted complexes. Similar structures have been computed for tris(butadiene)tungsten and for related, substituted systems. A structure redetermination of tris(butadiene)molybdenum at low temperature shows that the erroneous bond lengths obtained previously are due to the presence of a disorder resulting from the superposition of two different orientations of the three butadiene ligands with different site occupation factors. Refinement of this disorder results in a physically more plausible orientation of the anisotropic displacement parameter and gives by a factor of 10 improved estimated standard deviations for the geometrical features. A much better agreement between theory and experiment is attained. It is now obvious that resonance structures involving metallacyclopentene rings contribute significantly to bonding. This conclusion has been confirmed by natural bond orbital/natural resonance theory analyses, which indicate overall larger contributions from metallacyclopentene resonance structures than from traditional resonance structures with p-bonded diolefins. Explanations are provided for the trigonal prismatic structure preferences. MO analyses differ qualitatively and quantitatively from previous work, due to the use of refined structural parameters. Computed ligand NMR chemical shifts agree well with experimental data, provided that they are calculated at the correct structures.

Item Type: Article
Additional Information: Other co-authors: Stalke, D., Strohmann, C.
Schools and Departments: School of Life Sciences > Chemistry
Depositing User: Geoff Cloke
Date Deposited: 06 Feb 2012 20:40
Last Modified: 30 Nov 2012 17:07
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