Enthalpy of Co Dissociation From M(CO)6(m = Cr, Mo, W) In Alkane Solvent: Determination of Intermolecular Agostic Bond Strengths


The enthalpy of CO dissociation from M(CO)6(M = Cr, Mo, W) in alkane solvent has been determined by photoacoustic calorimetry (PAC). The reaction involves the substitution of an M-CO bond by an intermolecular agostic bond (M-alkane bond). The enthalpies for CO dissociation from Cr(CO)6in pentane, heptane, isooctane, and cyclohexane are endothermic by 28, 27, 26, and 24 kcal/mol, respectively. The difference in enthalpies for heptane and cyclohexane is significant and is attributed to a stronger agostic bond to cyclohexane. It is concluded that agostic bond formation is preferred with secondary CH bonds relative to primary. The enthalpies for CO dissociation from Mo(CO)6and W(CO)6in heptane are endothermic by 32 and 33 kcal/mol, respectively. From literature M-CO bond energies and the above enthalpies the corresponding agostic bond strengths with heptane can be calculated yielding 10, 9, and 13 kcal/mol (Cr, Mo, and W, respectively). The significance of these results and the role of agostic bonding in the reactions of metal carbonyls are discussed. © 1989, American Chemical Society. All rights reserved.

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