Bond-Energy Trends

Extensive reviews of the effects of fluonnation on stmeture and bonding are available [75, 76, 77], and only the charactenstic trends in bond strengths will be covered here. The bond energies cited are average values corrected for the revised heats of formation of alkyl radicals [78], but their precision is seldom better than 2 kcal/mol for the fluoro compounds. 

In general, it was found that this process is strongly endothermic for sulfur diimides, approximately thermoneutral for selenium diimides and strongly exothermic for tellurium diimides, consistent with experimental observations. These differences can be attributed to the expected trend to lower r-bond energies for chalcogen-nitrogen (np-2p) r-bonds along the series S (n = 3), Se (n = 4) and Te (n = 5). 

It is interesting that the bond energy relative to the bond-forming state of the atoms shows the same monotonic trend for the alkaline-earth metals as for the alkali metals. The irregularity in the heats of sublimation at magnesium is due to the high... 

Assuming an approximately constant cohesive energy per C-C bond, that trend is understandable. With clusters on the above general type, the number of carbon atoms is 6N, the number of dangling bonds is 6N, and the number of C-C bonds is 9N -3N. The energy per bond shows a smoother trend, the numbers being 71.0, 77.6 and 79.9 kcal/mol, respectively. Alternatively, the energies can be fitted to a two-parameter expression of the form... 

C06-0059. Consult Table to find the single-bond energies for bonds between identical row 3 elements (for example, P—P). Is there a trend If so, use the trend to predict the bond energy of an A1—A1 bond. 

As described in Section 6-, energy must be supplied to break any chemical bond. Bond energies, like bond lengths, vary in ways that can be traced to atomic properties. There are three consistent trends in bond strengths ... 

Use molecular orbital diagrams to explain the trend in the following bond energies B2 = 290 kJ / mol, C2 = 600kJ/mol, and N2 =942kJ/mol. 

The data show that bond energies for these three diatomic molecules increase moving across the second row of the periodic table. We must construct molecular orbital diagrams for the three molecules and use the results to interpret the trend. 

A proton transfer reaction involves breaking a covalent bond. For an acid, an H — X bond breaks as the acid transfers a proton to the base, and the bonding electrons are converted to a lone pair on X. Breaking the H — X bond becomes easier to accomplish as the bond energy becomes weaker and as the bonding electrons become more polarized toward X. Bond strengths and bond polarities help explain trends in acidity among neutral molecules. 

DFT-GGA calculations are very useful for investigating plausible reaction pathways of various molecules on surfaces. The method provides detailed information on the bonding geometry, on bond energies as well on activation barriers, and transition states which are otherwise not accessible. Typical accuracies in such numbers amount to a few tenths of an eV, making the method particularly useful to investigate trends. 

Figure 34 Trend in bond energies for loss of a ligand from [Ni(CO)J+ x— 1-4), [Ni(N2)J+ (x= 1-4), and [Ni(NO)J+ x— 1-3) (reprinted with permission from ref. 2409 1995, American Chemical Society).

In contrast to carbon compounds, force constants and bond distances of SiO and SiOz - as well as of SiS and SiS2 - are equal. On the other hand, the trend within the bond energies shows the expected decrease from SiO to Si02 and from SiS to SiS2, respectively. 

No data are available for compounds with C=Se and C=Te bonds and their equilibria with the seleno- and telluro-enol tautomer. For simple selenoacetone, its tautomer, and telluro-analogues, quantum mechanical calculations have been carried out using different methods.5 These calculations have shown that on passing from O to Te, the energy difference between the tautomeric forms is reduced, a trend that parallels the calculated C=E bond energies, which decrease from O to Te. 

Why is the bond energy of F2 less than that of Cl2 (This difference runs contrary to the trend between other first- and second-row diatomics.)... 

The vertical trend in d-block bond energies can be rationalized in terms of the relative sizes of s and d valence orbitals. As shown in Fig. 4.94, the d orbitals typically are contracted to lie inside the valence s orbital, but this disparity diminishes on going down a column as the d orbitals grow in size. This growth is physically... 

---