Stepwise bond dissociation enthalpies

To derive the chromium-benzene bond dissociation enthalpy, the values of several stepwise bond dissociation enthalpies in chromium hexacarbonyl are... 

The 72 kJ mol-1 discrepancy observed for Z)//°[(CO)3Cr-C6H6] alerts us to two important issues (1) Stepwise bond dissociation enthalpies are often very different from their mean (2) bond dissociation enthalpy values may not be transferable from one molecule to another. In other words, how can we assess the assumption that 3(Z)//°)(Cr-CO) in Cr(CO)6 is similar to Z)//j°(Cr-CO) + Z)// (Cr-CO) + DH (Cr-CO) in Cr(CO)3(C6H6) This question will be discussed in section 5.3. With regard to the former issue, the data in figure 5.4 provide further illustration for some first-row hydrides. The values shown in the plot are results from ab initio calculations for the species involved, in excellent agreement with the most accurate experimental data [78]. 

Figure 5.4 Stepwise bond dissociation enthalpies in several first-row hydrides. For each compound, the light gray bars, starting on the left, represent DH°(A-H), DH A-H), etc. The dark gray bars represent the respective mean bond dissociation enthalpies, (DH°) (A—H). Data at T = 0 from [78],...

Finally, it is noted that the concepts of mean and stepwise bond dissociation enthalpies can also be defined in solution (see discussion in section 5.1). 

We have shown that the result of replacing stepwise bond dissociation enthalpies by mean bond dissociation enthalpies and transferring bond dissociation enthalpies from one molecule to another can be deceptive The assumption that in Cr(CO)3 (C6H6), DH° (Cr-CO) + DH° Cr-CO) + DH° Cr-CO) 3 (DH°)( Cr CO) led to an error of 72 kJ mol-1 in DH°[(CObCr-CeHe]. Yet this error cancels out if the same procedure is applied to derive relative Cr-arene bond dissociation enthalpies in a series of ( r 6-arcnc)chromium tricarbonyl complexes. 

The stepwise bond dissociation enthalpies Dj (Ti-Me) and D2(Ti-Me) in TiMe2Cp2 established from thermochemical data and extended Huckel calculations are compared to the respective halides and hydrides.34 The isolated C-H stretches of M(CH2D)2Cp2 (M = Ti, Zr, Hf) are lower than any other studied methyl complexes and the C-H bonds in the hafnium species are the longest and weakest yet characterised by this method.35... 

Now suppose that no data were available for the stepwise Cr-CO bond dissociation enthalpies and we wanted to estimate DIt° (CO)3Cr CV, IU ]. A simple approach starts by considering reaction 5.15, where all the Cr-CO bonds are cleaved. 

Although we ignore the values for some of the six stepwise Cr-CO bond dissociation enthalpies in chromium hexacarbonyl, their sum is equal (equation 5.16) to the enthalpy of reaction 5.15, which is calculated as 641.7 4.8 kJ mol-1 (at 298.15 K) from the well-known standard enthalpies of formation of all the species involved [16,17,31],... 

In Table 5 are also shown the 0 K stepwise Ag+—CO bond dissociation enthalpies of Ag(CO)4+ from Armentrout and his coworkers49. From their collision-induced dissociation experiments in a guided ion beam mass spectrometer, one finds a nonmonotonic change in ZJo[Ag(CO)n+—CO] as a function of n. That the dissociation energy increases from n = 0 to n = I has been explained in terms of 4s-3d[Pg.62]

In Table 5 are also shown the 0 K stepwise Ag+—CO bond dissociation enthalpies of Ag(CO)4+ from Armentrout and his coworkers . From their colhsion-induced dissociation experiments in a guided ion beam mass spectrometer, one finds a nonmonotonic change in Do[Ag(CO) CO] as a function of n. That the dissociation energy increases from n = 0 to n = 1 has been explained in terms of 4s-3da hybridization Z)o[Ag+—CO] is small because it is reduced by the energy needed to hybridize the metal ion, but Do[Ag(CO) "—CO] assumes a more conventional value because the Ag(CO)+ fragment aheady has the metal in the required hybridization. In turn, Z)o[Ag(CO)+—CO] > Do[Ag(CO)2 —CO] because the third carbonyl Mgand imphes loss of the 4s-3da hybridization. FinaUy, Do[Ag(CO)2+-CO] > Do[Ag(CO)3+-CO] is explained by an increase of hgand repulsion. 

Before proceeding with the thermodynamic discussion of intermolecular C-H activation on the basis of data from Table 1, it is appropriate to make a few comments. The conclusion that (60) must be more exothermic than —44kJmor implies that Z)/f°(M-Me)+Z)//°(M-H)>483kJmol . It must be noted, however, that these bond dissociation enthalpies refer to process (65) and that there are no values in Table 1 (or elsewhere) that can be assigned to this stepwise reaction ... 

The mean bond enthalpy for the stepwise dissociation of both O-H bonds in water is (499 + 428)72 = 463.5 kjmol-h... 

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