Thermochemistry of Silane Substitution of CO in Metal Carbonyl Complexes. Interaction of Silanes with Metal Centers


By using photoacoustic calorimetry, the enthalpy of CO substitution by triethylsilane for M(CO)6 (M = Cr, Mo, W), (η6-C6H6)Cr(CO)3, and η5-CpMn(CO)3 (Cp = cyclopentadienyl) in neat triethylsilane was found to be 16, 19, 18, 17, and 31 kcal/mol, respectively. The enthalpy of CO substitution by alkane solvent for (η6-C6H6)Cr(CO)3 (in heptane) and η5-CpMn(CO)3 (in cyclohexane) was found to be 33 and 45 kcal/mol, respectively. By using these data and metal-CO bond energies, the enthalpies of the metal-silane and metal-alkane binding in the adducts were estimated to be in the range of 21-28 and 9-13 kcal/mol, respectively. Thus, the variation in the binding energies is 7 and 4 kcal/mol, respectively, while the variation for the corresponding metal-CO bonds is 18 kcal/mol. These results suggest the metal-silane interactions are more like the corresponding metal-alkane interactions than metal-CO bonds. It is concluded that for the cases studied the silane undergoes an incomplete oxidative addition to the metal, and the metal-silane interaction is a three-center two-electron bond. Silane adducts from Cr(CO)6 and W(CO)6 have been detected by UV-vis and IR spectroscopy. The results suggest the M(CO)5(HSiEt3) may be more stable than previously presumed. © 1990, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society