Polar bonds, solid-state materials

Polar bonds, solid-state materials with. 276-288 Polarizability tensor, 67 Polarization of ions, 129-134... 

Solid-State Materials with Polar Bonds... 

In this review, we present a selection of studies from our own laboratory, intended to introduce a solid-state chemist to both the practical and theoretical considerations that need to be taken into account in XPS measurements of solids with substantial covalent character. Metal phosphides, arsenides, and antimonides represent such a category of solids where the bonding retains some polarity that notions of electron counting derived from the Zintl concept still prove helpful in providing a frame of reference for comparing charge distributions. We also describe the applications of XAS to complementary studies of the electronic structure of these materials. 

Organic solids have received much attention in the last 10 to 15 years especially because of possible technological applications. Typically important aspects of these solids are superconductivity (of quasi one-dimensional materials), photoconducting properties in relation to commercial photocopying processes and photochemical transformations in the solid state. In organic solids formed by nonpolar molecules, cohesion in the solid state is mainly due to van der Waals forces. Because of the relatively weak nature of the cohesive forces, organic crystals as a class are soft and low melting. Nonpolar aliphatic hydrocarbons tend to crystallize in approximately close-packed structures because of the nondirectional character of van der Waals forces. Methane above 22 K, for example, crystallizes in a cubic close-packed structure where the molecules exhibit considerable rotation. The intermolecular C—C distance is 4.1 A, similar to the van der Waals bonds present in krypton (3.82 A) and xenon (4.0 A). Such close-packed structures are not found in molecular crystals of polar molecules. 

It is now time to show how the ideas developed in the previous chapters can be applied to real chemical systems. Apart from a few simple gases, the materials we come across in everyday life are either solids or liquids. A proper understanding of the chemistry of the solid state requires some appreciation of the role of symmetry in crystals and is therefore deferred to Part III. This chapter explores the use of bond valences to understand the simpler chemistry of liquids. Most of this chapter is devoted to the chemistry of aqueous solutions because water is not only the solvent of choice for polar systems but also the most common solvent in our environment. 

The general intransigence of the crystalline iminolithium hexamers toward further interaction with Lewis bases is not shown by the amorphous diaryliminolithiums. This may reflect their extensively stacked nature (Section II,A Fig. 10), which, while raising the lithium coordination number to four in all but the outer rings of the polymer, will presumably weaken individual N—Li bonds. These materials dissolve quite readily in several polar solvents, e.g., THF (66), pyridine (66, 78, 85), and HMPA (86). Crystalline complexes can be recovered from these solutions. Two of these, both derivatives of (Ph2C=NLi) , have been characterized structurally in the solid state. The tetrameric cubane (Ph2C=NLipyridine)4 (7) is depicted in Fig. 14 (78, 85). 

---