Bond enthalpy contributions

A force field that can produce vibrational spectra has a second advantage in that the Ay// calculations can be put on a much more satisfactory theoretical base by calculating an enthalpy of formation at 0 K as in ab initio procedures and then adding various thermal energies by more r igorous means than simply lumping them in with empirical bond enthalpy contributions to Ay//-. The stronger the theoretical base, the less likely is an unwelcome surprise in the output. 

Table 6.3 Some enthalpies of atomization (Af/j, 298 K) and comparative bond-enthalpy contributions, E...

Metal-Hydrogen Bond Enthalpy Contributions. Metal Carbonyl... 

Note The bond enthalpy contributions (b.e.cs) are shown in the tables to the nearest whole kilojoule (kJ). 

Table 1. Standard enthalpy of formation of metal carbonyls [Mm(CO) ] in the gas phase. Bond description and bond enthalpy contributions (T, M and B) to the enthalpy of disruption, AHq...

Fig. 1. Variation of the terminal metal-CO bond enthalpy contribution, T (M-CO) kj mol as a function of the enthalpy of atomization of the metal, AHf (M, g) kJ mol-1...

Table 10. Bond enthalpy contributions, D(M-ArH) kJ mol, in bis(arene) metal compounds...

While these disadvantages are severe, electron impact determinations play a useful role in suggesting the pattern of variation in bond enthalpy contributions in molecules which have not been studied by conventional thermochemical techniques. A few examples of this are shown in Table 11. Electron impact measurements also indicate that D (Ru-Cp) in ruthenocene is ca. 100 kJ mol-1 greater than D (Fe-Cp) in ferrocene82). 

Table 12a. Standard enthalpy of formation, A/ff(g), enthalpy of disruption, AHjy, and metal-halogen bond enthalpy contribution, (M-X), in metal carbonyl halides (kJ mol-1)...

Table 14. Standard enthalpies of sublimation, formation and disruption and bond enthalpy contributions, iT(W-N) kJ mol-1, for N-donor complexes of tungsten W(CO)6 nLn]...

Table 15. Enthalpy of sublimation, formation and disruption (kJ mol-1) for (jr-arene M(CO>3] compounds and arene-M bond enthalpy contributions IT(ArH-M) kJ mol-1...

They are formally isoelectronic with the (ArH)Cr(CO)3 series, and are derived from Co4(CO)12. The thermal decomposition of three representatives of the series has been studied by microcalorimetry84) and the results are shown in Table 16. Once again heats of sublimation have had to be estimated by comparison with the chromium analogues. The enthalpy disruption can be divided by taking T = 134 kJ mol-1 (Table 1) so that the b.e.c of the [Co4(CO)9] fragment in Co4(CO)i2 is 1722 kJ mol-1. The (ArHCo) bond enthalpy contribution is then obtained in the usual way the results are shown in Table 16. It is clear that as in the chromium series, the b.e.c (ArH-Co) increases along the series benzene < mesitylene < hexamethylbenzene. 

Table 16. Enthalpy of formation, A//f (c) and disruption, AHjy and bond enthalpy contribution, i (Co-L) in LCo4(CO>9. All values in kJ mol-1 (Ref.84))...

Table 17. Enthalpy of sublimation/vaporization, A//sub/vac, enthalpy of disruption, A/fo, and olefin-iron bond enthalpy contribution for [Fe(CO)n(olefin),j] compounds. All values are in kJ mol-1...

Table 18. Comparison of (M-C2H4) and (M-CO) bond enthalpy contributions for M = Fe, Ni and Rh...

Table 23. Selected nickel-ligand bond enthalpy contributions, D[(CpNi-L)+] kJ mol-1 determined by ion cyclotron resonance (Ref.93 )...

It is to be hoped that measurements will be made in the near future which will put more substantial flesh on the skeleton of known bond enthalpy contributions in organo-transition metal compounds, so that a better understanding of the energetics of reactions such as olefin disproportionation (metathesis) and hydroformylation may be achieved. 

Table 26. Selected bond enthalpy contributions, D (M-L) and (M-L) kJ mol-1, in organometallic compounds of chromium, manganese, iron cobalt and nickel and related compounds (An asterisk ( ) denotes the average value in a series)...

There are alternative ways of viewing the previous problem that are closer to the idealized concept of chemical bond strength. Consider reaction 5.20, where all the chromium-ligand bonds are cleaved simultaneously. The enthalpy of this disruption reaction at 298.15 K, calculated as 497.9 10.3 kJ mol-1 by using enthalpy of formation data [15-17,31], can be given as a sum of three chromium-carbonyl and one chromium-benzene bond enthalpy contributions (equation 5.21). 

Apparently, there is not much advantage in using bond enthalpy contributions to discuss bonding energetics in a series of similar complexes. As already stated, we could have selected any value for Z)//,°(Cr-CO) + DH (Cr-CO) + Z)//j (Cr-CO) and then derived chromium-arene bond dissociation enthalpies in Cr(CO)3(arene) compounds, all based on the same anchor. The trend would not be affected by our choice. Nevertheless, besides emphasizing that the absolute values so obtained should not be regarded as bond dissociation enthalpies, the bond enthalpy contribution concept attempts to consider a pertinent issue in molecular energetics the transferability of bond enthalpies. 

Figure 5.5 Thermochemical cycles relating O-H bond enthalpy contributions ( s) with bond dissociation enthalpies (DH°) in phenol and ethanol. ER are reorganization energies (see text).

In summary, the previous example shows that bond dissociation enthalpies should not be correlated with bond lengths unless the relaxation energies of the fragments are comparable. On the other hand, when two bonds between the same pairs of atoms have identical bond lengths, it is sensible to assume that they have similar bond enthalpy contributions. Hence, in this case, a bond enthalpy contribution can be transferred from one molecule to another. 

There are many other examples in the literature where the concept of bond enthalpy contribution (either E or Es) has been applied. Let us return to the case of Cr(CO)3(C6H6) and examine the procedure to estimate E sjCr-CeHe). This is much more complex than the case of the O-H bond in phenol and ethanol, as suggested by figure 5.6. 

Let us concentrate on the thermochemical cycle of figure 5.6 that involves the disruption of the complex Cr(CO)3(C6H6). The enthalpy of this reaction, previously calculated as 497.9 10.3 kJ mol-1 from standard enthalpy of formation data, can be related (equation 5.24) to the bond enthalpy contributions EsfCr-CO ) andE s(Cr (V.He) through the reorganization energies ER(C() ) and ER(C(tHf )- Two asterisks indicate that the fragment has the same structure as... 

Figure 5.6 Thermochemical cycles to estimate the Cr-CgHg bond enthalpy contribution (fs) in Cr(CO)3(C6hl6). ER are reorganization energies. One asterisk indicates that the fragment has the same structure as in Cr(CO)6, and two asterisks mean that the fragment has the same structure as in Cr(CO)3(CgH6)-...

We are now left with the evaluation of E s (Cr—CO), the Cr-CO bond enthalpy contribution in Cr(CO)6. The third thermochemical cycle in figure 5.6 shows how this bond enthalpy contribution can be evaluated from the Cr-CO mean bond dissociation enthalpy (107.0 0.8 kJ mol-1 see section 5.2) and the reorganization energy ER(CO ). 

---