Ionicity of bonds

A large number of binary AB compounds formed by elements of groups IIIA and VA or IIA and VIA (the so-called III-V and II-VI compounds) also fcrystallize in diamond-like structures. Among the I-VII compounds, copper (I) halides and Agl crystallize in this structure. Unlike in diamond, the bonds in such binary compounds are not entirely covalent because of the difference in electronegativity between the constituent atoms. This can be understood in terms of the fractional ionic character or ionicity of bonds in these crystals. 

The x-ray spectroscopic method for investlgatii the chemical bonding in compounds is free of these limitations. Therefore, there is a continually growing use of x-ray spectra in studies of many aspects of the chemical bonding in solids. One such problem is the determination of the degree of ionicity of bonds (the charge of an element in a compound) from the chemical shifts of the x-ray emission lines. 

The problem of the determination of the degree of ionicity of bonds is particularly important because in many solids, such as semiconducting compounds and alloys, we encounter mix-li of the ionic, the covalent, and the metallic bonding. 

We determined the degree of ionicity of bonds in some compounds of phosphorus with zinc and cadmium by measuring the relative shifts of the x-ray emission lines of... 

On the example of Ho it has been shown that the addition of coordinative ligand (NH3, py, C HijNC) increases the ionicity of bond between the metal and polyene substituents [28]. The shift of the deformation modes frequency of C-H bonds to the long-wave region testifies this fact in the spectra of adducts. 

Analysis of electronic Wavefunctions usually commences with the evaluation of atomic charges. Playing a paramount role in the description of electronic structures of molecules, these quantities not only measure ionicity of bonds but also serve as reactivity indices for reactions that involve charged species. In addition, being the lowest-rank multipoles, atomic charges are often used in approximate computations of several electronic properties, including the far-field electrostatic potential. ... 

A critical discussion of this approach is possible under the light of the general considerations of section I. VS2 and TiS2 are favorable cases fi om the point of view of both the ionicity of bonds and the d-sp redox competition. 

Ayotte P, Bailey C G, Weddle G FI and Johnson M A 1998 Vibrational spectroscopy of small Br (Fl20) and I Fl20) clusters infrared characterization of the ionic hydrogen bond J. Phys. Chem. A 102 3067-71... 

Meot-Ner M 1984 Ionic hydrogen bond and ion solvation 2. Solvation of onium ions by 1-7 water molecules. Relations between monomolecular, specific and bulk hydration J. Am. Chem. Soc. 106 1265-72... 

A large number of ordered surface structures can be produced experimentally on single-crystal surfaces, especially with adsorbates [H]. There are also many disordered surfaces. Ordering is driven by the interactions between atoms, ions or molecules in the surface region. These forces can be of various types covalent, ionic, van der Waals, etc and there can be a mix of such types of interaction, not only within a given bond, but also from bond to bond in the same surface. A surface could, for instance, consist of a bulk material with one type of internal bonding (say, ionic). It may be covered with an overlayer of molecules with a different type of intramolecular bonding (typically covalent) and the molecules may be held to the substrate by yet another fomi of bond (e.g., van der Waals). 

When an element has more than one oxidation state the lower halides tend to be ionic whilst the higher ones are covalent—the anhydrous chlorides of lead are a good example, for whilst leadfll) chloride, PbCl2, is a white non-volatile solid, soluble in water without hydrolysis, leadflV) chloride, PbC, is a liquid at room temperature (p. 200) and is immediately hydrolysed. This change of bonding with oxidation state follows from the rules given on p.49... 

X-ray structural studies have played a major role in transforming chemistry from a descriptive science at the beginning of the twentieth century to one in which the properties of novel compounds can be predicted on theoretical grounds. When W.L. Bragg solved the very first crystal structure, that of rock salt, NaCl, the results completely changed prevalent concepts of bonding forces in ionic compounds. 

On the other hand, fluorine s high electronegativity and its ability to form mostly ionic chemical bonds, provide materials with several useful properties. First, compared to oxides, fluoride compounds have a wide forbidden zone and as a result, have low electroconductivity. In addition, fluorides are characterized by a high transparency in a wide optical range that allows for their application in the manufacturing of electro-optical devices that operate in the UV region [42,43]. 

In Chapter 6 the element hydrogen was characterized as a family by itself. Often its chemistry distinguishes it from the rest of the periodic table. We find this is the case when we attempt to predict the ionic character of bonds to hydrogen. 

The ionization energy of the hydrogen atom, 313.6 kcal/mole, is quite close to that of fluorine, so a covalent bond between these two atoms in HF is expected. Actually the properties of HF show that the molecule has a significant electric dipole, indicating ionic character in the bond. The same is true in the O—H bonds of water and, to a lesser extent, in the N—H bonds of ammonia. The ionic character of bonds to hydro-... 

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

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