Compound energetics trends

Most energetic contributions are, as we have discussed, difficult to predict and large experimental efforts have for that reason been devoted to derive systematic trends in the energetics of classes of materials. In this chapter we will try to convey an overview of periodic trends in the thermodynamic properties of inorganic compounds and we will also present selected examples illustrating some of the more usual rationalization schemes. Finally, trends in enthalpy of mixing are treated. Also here we aim to look at trends and rationalization schemes. The chapter is by no means exhaustive - only selected classes of compounds and selected rationalization schemes are discussed. 

---

Two elements or compounds that do not adopt the same crystal structure cannot exhibit complete solid solubility except when one of the space groups is a subgroup of the other. The energetics of solid solutions of compounds with different structures are obviously difficult to treat systematically and trends may be impossible to obtain, since the energetics is largely related to structural short-range order. We will thus confine our discussion of solid solutions to systems where the two end-members take the same crystal structure. 

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. 

The same trends are observed for the change from siloxanes to carbosilanes (7—>12). The higher surface tension of the surprisingly polar carbosilane is caused by an increased portion Obviously methyl groups attached to flexible siloxane backbones are energetically distinctly different from those bonded to rigid structures. Expectedly a significant portion occurs at the interface solid/liquid for compound 12... 

---