AN INVESTIGATION OF THE HOMOLYTIC DISSOCIATION OF [ETA-5-C5ME5CR(CO)3]2 AND RELATED COMPLEXES - THE ROLE OF LIGAND SUBSTITUTION ON THE SOLUTION THERMOCHEMISTRY OF METAL METAL BOND-CLEAVAGE

Thermodynamic parameters for dissociation of the metal-metal bonded dimers [eta-5-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 {eta-5-C5H5Cr(CO)2(PR3)} (R = Ph, Et), no sign of dimer formation is found even at -80-degrees-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 [eta-5-C5R5Cr(CO)3]2 forming Hg[eta-5-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 [eta-5-C5Me5Cr(CO)3]2 as well as its structure as determined by X-ray crystallography. The Cr-Cr distance, 3.3107 (7) angstrom, is longer than that reported for [eta-5-C5H5Cr(CO)3]2; however, the increased tendency to form radicals for this complex is due more to entropic than enthalpic factors.