Mean bond energies

Strictly, these values are bond enthalpies, but the term energies is commonly used. Other descriptions are average standard bond energies, mean bond energies . 

Figure 17.9 Mean bond energies of halogen fluorides.

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). 

A general survey has been given by Long (158), who points out that enthalpy regularities would only be expected over restricted areas of the periodic table. Discontinuities would be expected where an increase in coordination number occurred between Periods 4 and 5, or where the relative stability of valence states changed, or where ion configurations were stabilized. Examples of trends in mean bond energies are presented in Section V,C. 

The mean bond energy Z (S—F) for SF6 can be regarded as a single bond energy and its transferability to ternary compounds can be examined in S—N—F compounds (see Table XX). 

Since the proton is centred in the bifluoride ion, it is possible to redefine the hydrogen-bond energy in a way comparable to the mean bond energies... 

Much of the enthalpy of formation data for the dialkylzincs was reported from two separate sources in 1949 . From these measurements each group derived the mean bond dissociation energies of the diaUcylzinc compounds. From Reference 12, the mean C—Zn bond energies were 172 kJmor for MeaZn and 150 kJmoU for EtaZn (and by assumption for the n-Pr2Zn and n-Bu2Zn species also). From Reference 18, the mean bond energies for the MeiZn and Et2Zn compounds were reported as 171 and 144 kJmoU, respectively. 

Unfortunately, there is sometimes confusion over what exactly we mean by bond energy. We could define a mean bond energy E as the contribution of an A—B pair to the total energy of the molecule ABn. Thus, for the gas methane (CH4),... 

Clearly, one should not use tabulated bond energy data for polyatomic molecules without knowing whether they are mean bond energies or bond dissociation energies. In any event, the relationship with the heat of formation of CH4 can be illustrated by an enthalpy cycle ... 

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,... 

Table 6.2 Mean bond energies from atomisation enthalpies at 25 °C of gaseous molecular substances, in kJ molRanges of values are not given in cases where the relevant data are available only for one or two molecules...

As far as the data permit, the dependence of bond energies on the hybridisations of the atoms concerned is allowed for. In the case of F—P, Cl—P and Br—P, the bond energies are based upon data for molecules in which the P atom has (approximately) sp3 hybridisation, e.g. PX3, PX2Y, POX3. These values are inappropriate for PX5 species, or for PX3Y2 etc. The mean bond energies found for, e.g., PF5 are considerably lower than for trivalent phosphorus species. 

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. 

The recommended enthalpies of formation of the gaseous trifluorides derived from these values are shown in table 22. From these values the mean bond energy can be derived as one third of the energy of the reaction ... 

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). 

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