Ionicity/ionic character definition

These definitions are clear, but they do not apply to the vast majority of real molecules in which the bonds are neither purely ionic nor purely covalent. Lewis recognized that a pair of electrons is generally not shared equally between two electrons because the atoms generally have different powers of attracting electrons, that is, they have different electronegativities, giving charges to both atoms. Such bonds are considered to have some covalent character and some ionic character and are known as polar bonds. 

At present, the most widely used definition of fractional ionic character in solid state chemistry is that of Phillips (1970), based on a spectroscopic approach. Phillips defined fractional ionic character as... 

However, starting from the same experimental evidence, Stewart et al. (1980) adopt the following definition of fractional ionic character ... 

Application of this definition to various compounds (in the gas phase) gives the results shown in Fig. 13.12, where percent ionic character is plotted versus the difference in the electronegativity values of X and Y. Note from this plot that ionic character increases with electronegativity difference, as expected. However, none of the bonds reaches 100% ionic character, even though compounds with the maximum possible electronegativity differences are considered. Thus, according to this definition, no individual bonds are completely ionic. This conclusion is in contrast to the usual classification of many of these compounds (as solids). All the compounds shown in Fig. 13.12 with more than 50% ionic character are normally considered to be ionic solids. Recall, however, that the results in Fig. 13.12 are for the gas phase, where in-... 

In considering the computational aspects of a local-MP2 treatment of electron correlation in periodic systems, Pisani et al point to number of distinctive features including (i) full exploitation of point symmetry (ii) proper definition of the local-virtual basis set (in) assessment of the locality of excitations concept (iv) proper use of different levels of treatment of 2-electron integrals (v) extrapolation of local results to infinity. Pisani et al. examine the relative importance of different kinds of local excitations and their dependence on the prevailingly covalent or ionic character of the crystal. They also demonstrate the usefulness of a multipolar approximation for the evaluation of the majority of 2-electron integrals which arise. 

To justify such a description within quantum mechanics we are led into a consideration of ionic and covEilent contributions to an approximate wave function. If covalent contributions are minor, the bond is said to be ionic in character, and the electrostatic model is considered to be applicable. Unfortunately, the terms ionic character and covalent character are used with various meanings. This is so, in part, because the rapid development of chemical bond theory has caused a drift of the meanings of these terms over the past two decades. Pauling s definitions, as presented in his book (1585, p. 48), no doubt represent the intent of most workers as of 1940. He concluded that there is a covalent bond between two atoms X and Y if the dissociation energy of X—is the mean of the dissociation energies of X— X and Y— Y. If the dissociation energy of X— Y exceeds this mean, the excess is attributed to additional ionic character of the bond. This criterion furnishes the basis for his scale of electronegativity, and ionic character is inter-... 

As the chemist has gained his sea legs in the use of wave mechanics he has attempted to define covalent and ionic character in terms of wave functions. The new point of view is presented, for example, by Coulson, who maintains, there are two distinct definitions of a covalent bond (447, p. 145). He proceeds to describe the bond wave function, first by a molecular orbital type approximation, and then by a valence bond type approximation. In either case the approximate wave functions consist of linear combinations of atomic orbitals. Partial ionic bonding is revealed by the magnitudes of coefficients which imply asymmetry of electron distribution. 

Regardless of preference, we are faced with three definitions of the term ionic character, in terms of electronegativity, molecular orbitals, or valence bond wave functions. Any paper on the nature of the H bond must be evaluated in the light of what the writer meant when he wrote that paper. 

In discussing bonds between unlike atoms, it is convenient to associate with every atom a quantity, x, representing its electron-attracting power in a bond, such that the ionic character of a bond P—Q is determined by It is clear that the definition x cc (/p + 4p), in terms of ionisation potential and electron affinity, might be satisfactory for the ease with which P+Q and P"Q+ could be formed would depend (p, 88) on... 

Such a modified picture is definitely not contradictory to the formulations [Rb902] 5 e" and [Csn03] + 5 e used in Chapter IV.l., because the reduction of the net charges of the clusters does not mean a reduction of the free electron concentration. Indeed, this result is equivalent to an assumption used by Burt and Heine (47) to explain the low work functions of CS11O3 (see V.4.). These authors emphasize that the iimer part of the clusters due to the concentrated negative charges of the Onions are hi ly repulsive for electrons. Therefore, the free electrons are confined to the periphery of the clusters. This assumption is visualized by the very simple picture of an ionic character of the iimer part of each Cs atom in the cluster and a metallic character of the outer part of each Cs atom. 

Application of this definition to various compounds (in the gas phase) gives the results shown in Fig. 8.13, where percent ionic character is plotted versus the difference in the electronegativity values of X and Y Note from this plot that ionic character increases with electronegativity difference, as expected. However, none of the bonds reaches 100% ionic... 

Chemistry has a knack of using terms such as valency, electronegativity and bonding which have a multiplicity of meanings. In its broadest sense, valency has been used to describe the ability of elements to combine with others. Russell s book provides a thorough analysis of the history of valency [15]. A chemical bond is more precisely defined as the force which holds two chemical entities together, but the definition encompasses a duality which at its extremes is based on either electrostatic (ionic) or covalent bonding and in between a variable amount of covalent and ionic character. 

In most organometallic compounds the M —C bond has, to a significant degree, covalent character. Definite ionic character of this bond is generally manifested only in compounds of the alkali and alkaline earth metals. The ionic or covalent contribution to the bond depends on ionization potential of the metal, the size of a resulting ion, the ratio of the ionic charge to its radius, and tr-donor, (j-acceptor, 7c-donor, and 7c-acceptor properties of ligands and their structure. 

The generally accepted valence-bond structure of the molecule is given by Pauling [1]. The bonding orbitals have a small amount of s character and are intermediate between p and sp [1]. The profile and the size of the lone pair orbitals involved in this system were studied [2]. The ionic character of the 0-F bonds was determined [3], using the definition of ionicity i in [4] ... 

It should be noted the PIL and dlL systems have similarities within their physical properties. Namely, both these classes have ionic characters similar to the more conventional IL/molten salts. However, they also posses properties that are more closely related to those of traditional non-aqueous molecular solvents. Based on this we now need to expand the definitions presented in Fig. 7.1 to encompass these properties, and this is presented in Fig. 7.2. 

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