Ionic character of a bond

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. 

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 ionic character of a bond increases with increasing electronegativity difference between the bonding elements. 

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). 

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... 

Fig. 3-8.—Curve relating tbo amount of ionic character of a bond to the electronegativity difference of the two atoms. Experimental points, based upon observed values of the electric dipole moment of diatomic molecules, are shown for 18 bonds.

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. 

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).

Absolute value of the electronic charge Electric quadrupole moment of a nucleus Atomic nuclear quadrupole coupling constant Nuclear quadrupole coupling constant Ionic character of a bond Nuclear spin... 

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 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... 

This graph shows how the percent ionic character of a bond depends on the difference in electronegativity of the atoms that form it. Above 50% ionic character, bonds are mostly ionic. What is the percent ionic character of a pure covalent bond ... 

Estimate the percent ionic character of a bond from its dipole moment (Section 3.7, Problems 33-38). 

The percent ionic character of a bond can be approximated by the formula 16A + 3.5A, where A is the magnitude of the difference in the electronegativities of the atoms (see Fig. 3.7). Calculate the percent ionic character of HF, FdCl, FdBr, Fdl, and CsF, and compare the results with those in Table 3.7. 

Thus the ionic character of a bond between atoms of the same electronegativity is zero. 100% ionic character is expected only for electronegativity differences of infinite value. For this bond d should also approach infinity, e. g. the energy of dissociation should be infinitely high. 

Eq. (18) is formally identical with Eq. (14) for the ionic character of a bond A—B in a diatomic molecule. The difference is, however, that Qmol is not valid for one bond A—B in the molecule AB but only for consideration of the whole molecule. Our ideas may be illustrated by considering a molecule AB3 with three equal bonds... 

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