Ionic bond character, partial

Cova lent bond with. partial ionic character... 

Ionic Bonding, Partial Ionic Character, and Electronegativity... 

Chapter 2. Ionic Bonding, Partial Ionic Character, and Electronegativity Papers SP 23 to SP 27 255... 

When two atoms share electrons unequally, it means that the bond between them is polar. Another way to describe this is to say that the bond has partial ionic character. For the molecule AB, this is equivalent to drawing two structures, one of which is covalent and the other ionic. However, there are actually three structures that can be drawn... 

The average metal-ligand distances, Pr—Cl =2.72 A and Pr—0=2.353 A, are shorter than those calculated by summing the ionic radii. The average P—0 bond length of 1.487 A is slightly shorter than that observed for other phosphor-triamide [1.501 for 0P(NH2)3]. The P—N (1.64 A) possesses partial -bond character. 

Such an off-center covalent bond displays partial ionic character. 

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. 

Polarization is one of the reasons for the asymmetrical form of the water molecule, and also may be partially responsible for the non-linearity of H2S molecules. Polarization would lead to the pyramidal shape observed for the molecules NH3 and PH3, but it is very doubtful whether it can be held responsible for the asymmetrical form of molecules such as PC13 and SOa. In these molecules, the central ion is positive, if it is assumed that the bonds in these compounds are ionic, and since positive ions have not a large polarizability, the distortion of the molecule can scarcely be due to polarization effects. Indeed, we cannot continue to consider these compounds as purely ionic in character, but will find it necessary to explain their asymmetry on the basis of the homopolar bond (see Section 53). Even in hydrogen compounds such as H20 and NH3 we shall find we have to take into account their partial homopolar structure in order to arrive at a really satisfactory explanation of their structures. 

Instead of using this description of the bond as involving resonance between an extreme covalent bond H C1 and an extreme ionic bond H+Cl-, we may describe the bond as a covalent bond with partial ionic character, and make use of the valence line, writing H—Cl (or H—Cl )... 

These covalent radii are for use in molecules in which the atoms form covalent bonds to a number determined by their positions in the periodic table—carbon four, nitrogen three, and so on. It is found empirically that the radii are applicable to covalent bonds with considerable ionic character for extreme ionic bonds, however, ionic radii are to be used (Chap. 13), and in some molecules, discussed in later sections, the partial ionic character plays an important part in determining the interatomic distances. 

This amount of double-bond character is to be expected from consideration of the principle of electroneutrality (Sec. 8-2). The 30 percent partial ionic character of the Si—Cl bond that corresponds to the difference in electronegativity of the atoms would place the charge + 1.2 on the silicon atom in the SiCU molecule. This electric charge would be reduced to zero if each bond had 30 per cent double-bond character, or to +0.2 (a value approximating electroneutrality) if each bond had 25 percent double-bond character. This amount of doublebond character (and the same amount of partial ionic character for each bond) is given by resonance among the six equivalent structures of type B ... 

The Si—Cl distance in SiCIF is small—only 1.99 A, corresponding to 31 percent double-bond character. The increase over the amount for the other molecules is probably to be attributed to the release of bond orbitals by the largely ionic (70 percent) Si—F bonds, permitting the Si—Cl bond to assume the amount of double-bond character that completely neutralizes the transfer of electric charge corresponding to its normal partial ionic character. 

The bonds between chlorine, bromine, and iodine and the heavier fifth-group and sixth-group atoms seem to have little double-bond character. The observed bond lengths are approximately equal to the calculated single-bond values (with the correction for partial ionic character, as given in Section 7-2) for example, observed for PC18, 2.043 A (calculated 2.03 A) for AsC13, 2.161 A (2.17 A) for SCI, ... 

It is interesting that the observed distance 1.271 A in the gas molecule CF corresponds to 40 percent of double-bond character, approximately enough to reduce the electric charge on the atoms (caujed by the 43 percent of partial ionic character of the single bond) to zero. 

For CFi this amount of double-bond character (19 percent), with the amount of partial ionic character of the C—F single bond (43 percent) corresponds to the electric charge +0.96 on the carbon atom Hence we may say that this molecule contains a C+ atom, with one negative charge resonating among the four fluorine atoms. As a first approximation the structure may be described as a resonance hybrid of the 12 structures of type A ... 

However, many covalent bonds do not equally share electrons such covalent bonds, as pointed out above, are referred to as polar covalent bonds or bonds of partial ionic character. Electronegativity is rated on a relative scale ranging from 4 (most electronegative, fluorine) to 0.7 (least electronegative, cesium) (Table 1.7). In general, the greater the difference in electronegativity between two elements, the more ionic will be the bond between them (Fig. 1.2). 

---