Ionic character in a bond

Needless to say, if ionic character affects the energy stability of a chemical bond it also affects the chemistry of that bond. The tendency toward minimum energy is one of the factors that determine what chemical changes will occur. As a bond becomes stronger, more energy is required to break that bond to form another compound. Hence we see that ionic bonds are favored over covalent bonds and that ionic character in a bond affects its chemistry. 

Electronegativities have important uses in chemistry in addition to predicting the amount of ionic character in a bond. The bond stretching force constant k (in units of 105 dynes cm-1) can be estimated for stable molecules exhibiting their normal covalences by the expression ... 

Electronegativity difference, AEN, is a measure of the degree of ionic character in a bond. 

The degree of ionic character in a bond is a function of the difference in electronegativity between the two bonded atoms. Figure 9.5 lists electronegativity values of the elements. The bonds in order of increasing ionic character are N-N (zero difference in electronegativity) < S-0 (difference 1.0) = Cl-F (difference 1.0) <... 

Pauling proposed the following correlation between electronegativity difference and percent ionic character in a bond ... 

From the PauHng s equation for calculating the percentage of ionic character in a bond, it has been observed that the Si—C, Al—C, Mg—C and Na—C bonds have approximately 12, 22, 34 and 45% ionic characters, respectively. Merely 6% ionic character in C—Cl bonds, in the case of chloro- organic compounds, affords many reactions, which indicates that even a small ionic character in this bond affects the reactivity of compounds. However, the increase in the stabiHty of carbanion R— does not increase the reactivity of organometallic compounds of a particular metal. 

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-... 

When the difference in electronegativities is great, the orbital may be so far over to one side that it barely covers the other nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion, leaving us with basically only one type of bond in organic molecules. Most bonds can be considered intermediate between ionic and covalent. We speak of percent ionic character of a bond, which indicates the extent of electron-cloud distortion. There is a continuous gradation from ionic to covalent bonds. 

FIGURE 3.10 The variation in percent ionic character to a bond and the difference in the electronegativities of the atoms. 

Having shown that the weighting coefficient (A) of the term giving the contribution of an ionic structure to the molecular wave function is related to the dipole moment of the molecule, it is logical to expect that equations could be developed that relate the ionic character of a bond to the electronegativities of the atoms. Two such equations that give the percent ionic character of the bond in terms of the electronegativities of the atoms are... 

The percent ionic character of a bond is based on the difference in electronegativity of its constituent atoms and Figure 11.7. 

The failure of Pauling s criterion for the fraction of ionic character of a bond (/ijer) in the case of alkali halides stems from the fact that the criterion fails to include the far from negligible polarization deformation of the ions in these completely ionic substances Rittner, Ref. 17, p. 1035). 

Experiments with 90 percent enrichment in the 1 or 2 position of 1,3 butadiene confirm earlier work that the live end is predominantly a 1,4 unit with a trans/cis ratio of /l The Li is bound to the Q( carbon of the 1,4 butadiene unit in what appears to be a highly localized 0 bond However, the presence of partial ionic character in the bond cannot be ruled out There is no evidence of Li being gf bonded to the carbon When a chelating diamine such as dipiperidyl ethane is added to the live cement, a drastic change takes place in the spectrum which suggests complete conversion to a delocalized ionic bonding. 

The amount of ionic character of a bond in a molecule must not be confused with the tendency of the molecule to ionize in a suitable solvent. The ionic character of the bond is determined by the importance of the ionic structure (A+B ) when the nuclei are at their equilibrium distance (1.275 A for HC1, for example), whereas the tendency to ionize in solution is determined by the relative stability of the actual molecules in the solution and the separated ions in the solution. It is reasonable, however, for the tendency toward ionization in solution to accompany large ionic character of bonds in general, since both result from great difference in electronegativity of the bonded atoms.4... 

The exponential term is a Thomas-Fermi screening factor which accounts for the screening by the core electrons. Direct measurement of the ionic character of a bond is a complex operation. In principle, a number of techniques such as X-ray or neutron diffraction, nmr, photoelectron or Mossbauer spectroscopy provide information about electron distribution and charge density in practice the results are usually far from unambiguous. 

This approach, which is certainly not exact, breaks down badly if extended to predominantly covalent bonds. To evaluate the degree of ionic character in such bonds if they occur in simple molecules, Pauling has used dipole moments. The term dipole has already been applied to the water molecule, a molecule in which there is a separation between the centers of positive and negative charge (Chap. 4). When the charges are equal in magnitude, the dipole moment, ju, is defined... 

Figure 1.2. Relationship between the ionic character of a bond and the difference in electronegativity of the bonded atoms (see also Table 1.7) (after Masterson et al., 1981, with permission).

Therefore, in order to solve the questions concerning the degree of ionic character of a bond, its strength, etc., one uses the values of Pauling s electronegativity or absolute electronegativities which are shown in Table 2.1 (the values are taken from the Reference Book [5]). 

How well can we tell the difference between an ionic bond and a polar covalent bond The only honest answer to this question is that there are probably no totally ionic bonds between discrete pairs of atoms. The evidence for this statement comes from calculations of the percent ionic character for the bonds of various binary compounds in the gas phase. These calculations are based on comparisons of the measured dipole moments for molecules of the type X—Y with the calculated dipole moments for the completely ionic case, X+Y. We performed a calculation of this type for HF in Section 13.3. The percent ionic character of a bond can be defined as... 

The farther away two elements are from one another in the electronegativity scale (horizontally in Figure 11-11), the greater is the amount of ionic character of a bond between them. When the separation on the scale is 1.7 the bond has about 50% ionic character. If the separation is greater than this, it would seem appropriate to write an ionic... 

The data shown serve to illustrate the fact that the dipole moment does not indicate, by itself, the ionic character of a bond. Thus the moments of the GF and GI bonds are very close, but the contribution of the ionic form in CF is much greater n. /, n-.- than in Cl, as the interatomic... 

In biochemical teactions, an add Is a proton donor, whereas a base is a proton acceptor- In an add, the bond between the proton (H ) and the parent compound is an ionic bond. In a strong add the bond has markedly ionic character. In a weak acid the bond has a more covalent character. When a strong acid, such as HCl, is dissolved in water it dissociates almost completely. V aker adds, such as acetic add or propionic acid, dissociate only partially in water. After the parent compound loses its proton it acts as a base, because It can now readily accept a proton. 

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... 

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