An Investigation of the Homolytic Dissociation of[η5-C5Me5Cr(CO)3]2 and Related Complexes. The Role of Ligand Substitution on the Solution Thermochemistry of Metal-Metal Bond Cleavage

W. Carl Watkins, Tilman Jaeger, Colleen E. Kidd, Suzanne Fortier, Michael C. Baird, Gabor Kiss, Gerald C. Roper, Carl D. Hoff

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Abstract

Thermodynamic parameters for dissociation of the metal-metal bonded dimers [η5-C5R5Cr(CO)2(L)]2 (R = H, L = CO, P(OMe)3; R = Me, L = CO) have been studied by several techniques including variable-temperature FTIR spectroscopy, magnetic susceptibility, NMR spectroscopy, and solution calorimetry. Values for the Cr-Cr bond strength are in the range 11-15 kcal/mol. Results obtained via NMR techniques in this system disagree with all other results probably due to a multiplicity of populated states for the radical monomers. For the radicals {η5-C5H5Cr(CO)2(PR3)} (R = Ph, Et), no sign of dimer formation is found even at -80°C, indicating that the Cr-Cr bond strength is <8 kcal/mol for these compounds. Calorimetric measurements of the heats of reaction of Hg and [η5-C5R5Cr(CO)3]2 forming Hg[η5-C5R5Cr(CO)3]2 have been measured for R = H, Me and indicate average Hg-Cr bond strengths of 20.3 and 20.7 kcal/mol, respectively. Synthetic and spectroscopic details are described for [η5-C5Me5Cr(CO)3]2 as well as its structure as determined by X-ray crystallography. The Cr-Cr distance, 3.3107 (7) Å, is longer than that reported for [η5-C5H5Cr(CO)3]2; however, the increased tendency to form radicals for this complex is due more to entropic than enthalpic factors.

Original languageEnglish (US)
Pages (from-to)907-914
Number of pages8
JournalJournal of the American Chemical Society
Volume114
Issue number3
DOIs
StatePublished - Jan 1 1992

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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