Molecular solids VSEPR model

In spite of a lack of firm theoretical foundation the VSEPR model is widely applicable to molecular geometries and even to some solids. Occasionally it fails to predict the correct structure. 

In all of the above cases, the feometry is in afreement with the expectations of the valence shell electron pair repulsion (VSEPR) model of Gillespie and Nyholm [77]. The VSEPR model accounts for the molecular geometry in nearly all cases for main group elements in free molecules. Deviations are observed in solids because of the contribution from the lattice energy. For example, in SnO and red PbO, the coordination MO4E is square pyramidal, instead of seesaw expected by the VSEPR model [78,79]. 

The structures of the element trihalides EX3 are covered in a number of textbooks on structural inorganic chemistry (4, 5), and these will not be discussed in great detail here. It is, however, worth mentioning some of the salient structural features. In most cases, a molecular trigonal pyramidal EX3 unit consistent with VSEPR theory predictions is readily apparent in the solid-state structure, although there are usually a number of fairly short intermolecular contacts or secondary bonds present. A general description of the structures as molecularly covalent but as having a tendency toward macromolecular or polymeric networks is therefore reasonable. Only in the case of the fluorides is an ionic model appropriate. 

In the previous chapters, we discussed various models of bonding for covalent and polar covalent molecules (the VSEPR and LCP models, valence bond theory, and molecular orbital theory). We shall now turn our focus to a discussion of models describing metallic bonding. We begin with the free electron model, which assumes that the ionized electrons in a metallic solid have been completely removed from the influence of the atoms in the crystal and exist essentially as an electron gas. Freshman chemistry books typically describe this simplified version of metallic bonding as a sea of electrons that is delocalized over all the metal atoms in the crystalline solid. We shall then progress to the band theory of solids, which results from introducing the periodic potential of the crystalline lattice. 

Finally, it should be noted that the XeF molecule exhibits a definite tendency to donate a fluoride ion and form the XeFj cation, which is isoelectronic and isasmic-tural with IF] as expected from the VSEFR model. The structure of solid XeF is complex, wilh 144 molecules of XeF per unit cell however there are no discrete XeF molecules. The simplest way to view the solid is as pyramidal XeF cations extensively bridged by free fluoride ions. Obviously, these bridges must contain considerably covalent character. They cause the xenon-containing fragments to duster into tetrahedral and octahedral units (Fig. 6.ISa,b). There are 24 tetrahedra and eight octahedra per unit cell, packed very efficiently as pseudospheres into a CujAu structure (Fig, 6.ISc)."-i The structure thus provides us with no information about molecular XeF, but it does reinforce the idea that the VSEPR-correct, square pyramidal XeF] is structurally stable. 

---