Mean thermochemical bond energy

Figure S.l. Mean thermochemical bond energies Jor represenlaiive bonds in Group IV...

The Si—Si bond is weaker than the C—C bond (mean thermochemical bond energies are C—C in diamond, 356 kJ mol" Si—Si... 

Halides. Xenon duorides, xenon oxide tetraduoride [13774-85-1XeOE, and their complexes are the only thermodynamically stable xenon compounds. Xenon diduo ride [13709-36-9] xenon tetraduoride [13709-61-0], and xenon hexaduoride [13693-09-9] are colodess, crystalline soHds which can be sublimed under vacuum at 25°C. The mean thermochemical bond energies are XeE2, 132.3 0.7 kJ/mol (31.6 0.2 kcal/mol) XeE ... 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

XeF2 is a colorless crystalline compound stable up to 500°C, m.p. -129°C, with considerable vapor pressure for solids—4.5 mmHg (20°C). It is a linear symmetrical molecule. The mean thermochemical energy of the Xe—F bond in XeF2 is 132kJ/mol,3 which is less than the F—F bond energy—157.3 kJ/mol.4... 

It is difficult to assess how close the two sets of results really are. The first one evidently depends on the precision of the thermochemical data that have been used, namely, AHf and ZPE + Hj-— Hq- Equation (6.8), on the other hand, is accurate. It is perhaps our best means for testing sp carbon charges an error of 1 me in the evaluation of one of the carbons translates into an error of 0.5 kcal/mol in bond energy. Now, the two sets are too close to warrant revision of the procedure, yet we cannot endorse it for more than it is an acceptable approximation. For our needs, and for the time being, Eq. (6.8) solves the problem. Moreover, the reasoning is that if this approximation holds in the close neighborhood of nitrogen, it should be all the more acceptable for carbons in positions y, S, and so on. 

Surface Bond Energies Thermochemical data are very scant in the area of oxygen chemisorption (57). These data would be of great value for interpreting spectroscopic and kinetic data and for the analysis of reaction mechanisms. The vast majority of the available data are for low oxidation state systems (55). Although calorimetry offers a means for direct measurements, for analysis of reaction pathways it is necessary to have detailed values for many types of species (M-OH, MO-H, M-OR, M-R, M-O, M-H), and these are usually... 

The mean bond energies with the catalyst may be obtained from thermochemical measurements. By means of these and Equation (30) the author (2, S) had already some time previously calculated the sequence in the ease of rupture of bonds on Ni, namely,... 

Here we consider the factors which determine whether a given compound prefers an ionic structure or a covalent one. We may imagine that for any binary compound - e.g. a halide or an oxide - either an ionic or a covalent structure can be envisaged, and these alternatives are in thermochemical competition. Bear in mind that there may be appreciable covalency in ionic substances, and that there may be some ionic contribution to the bonding in covalent substances. Since there is no simple means - short of a rigorous MO treatment - of calculating covalent bond energies, and since quantitative calculations based upon the ionic model are subject to some uncertainties, the question of whether an ionic or a covalent structure is the more favourable thermodynamically cannot be answered in absolute terms. We can, however, rationalise the situation to some extent. 

Xe04 as a molecular substance is thermodynamically unstable, and highly explosive (as is Xe03). The bonds are thermochemically weak -the mean bond energy in Xe04 is only 88 kJ mol-1 - but the stretching force constant, obtainable from infra-red spectra and a measure of the intrinsic bond strength, is consistent with at least some double-bond character. 

Thermodynamic data in the area of transition metal chemistry is available, but additional studies would be desirable. One of the early indications that C—Ti bonds are not notoriously weak was obtained from the heats of combustion of Cp2Ti(CH3)2 and Cp2Ti(C6H5)2 with subsequent estimation of the a-bond dissociation energies (250 kJ/mol-1 and 350 kJ/mol 1, respectively)49. From heats of alcoholysis of a number of titanium, zirconium and hafnium compounds, and heats of solution of the products as well as subsidiary data, Lappert estimated heats of formation (AHf°) and thermochemical mean bond energy terms (EM X) of metal—X bondsso> (Table 2). 

Table 3 Mean thermochemical bond energies of some main group fluorides ...

Thus, in the first place, by means of the combined thermochemical and comparative methods one obtains values for bond energies of hj drogen, carbon, and oxygen with nickel (Table XI) very similar to those found when using the first variant of the kinetic method for chromia (Table XII). At the same time, as the chemical compositions of nickel and chromia catalysts are different these values do not coincide, which is consistent with the theory. 

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