Partial ionic character of bonds

The discussion of interatomic distances is less simple for intermetallic compounds than for pure metals among the complicating factors are the partial ionic character of bonds, the transfer of electrons, with consequent changes in valency, and the preferential use of the valencies of an atom in the formation of strong bonds rather than weaker ones. These factors, which of course participate in minimizing the energy of the system, usually operate to decrease the interatomic distances. Then-effects may be illustrated by some examples. 

Contribution of Unshared Electron Pairs to the Electric Dipole Moments of Molecules.—In the preceding chapter we discussed the dipole moments of molecules, in relation to the partial ionic character of bonds, without considering the possible contribution of unshared electron pairs. A simple treatment based on hybrid orbitals provides some justification of this procedure. 

Relation between Electronegativity Difference and Amount of Partial Ionic Character of Bonds... 

Figure 9.21 AEN ranges for classifying the partial ionic character of bonds.

In the fourth paper of the series, Pauling (1932a) went a step further in the discussion of the partial ionic character of bonds. To the qualitative criterion for bond character presented before, he added a quantitative semiempirical criterion that enabled him to determine the approximate percentages of ionic and covalent character of bonds and then to map atoms in a scale of relative electronegativities. Pauling was thus able to suggest a viable alternative to the quantum mechanical treatment outlined in the former paper, which, as he pointed out before, was impossible to carry out except in the simplest cases. 

The theory as developed between 1852 and 1916 retains its validity. It has been sharpened, rendered more powerful, by the modem understanding of the electronic structure of atoms, molecules and crystals but its character has not been greatly changed by the addition of bond orbitals, the theory of resonance, partial ionic character of bonds in relation to electronegfativity, and so on. It remains a chemical theory, based on the tens of thousands of chemical facts, the observed properties of substances, their stmcture, their reactions. It has been developed almost entirely by induction (with, in recent years, some help from the ideas of quantum mechanics developed by the physicists). It is not going to be overthrown. (Pauling 1970, 998, emphasis ours)... 

The simplest explanation for the hydrogen bond is based upon the polar nature of F—H, O—H, and N—H bonds. In a molecule such as H20, the electron pair in the O—H bond is displaced toward the oxygen nucleus and away from the hydrogen nucleus. This partial ionic character of the O—H bond lends to the hydrogen atom some positive character, permitting electrons from another atom to approach closely to the proton even though the proton is already bonded. A second, weaker link is formed. 

In the foregoing treatment the assumption of additivity of interatomic distances in the compounds under discussion has been tacitly made. Examination of Table IV shows that this assumption is approximately substantiated by experiment. The agreement between the observed distances and the calculated radius sums is excellent in most cases. Aside from those just discussed, the exceptional crystals are AIN and SiC with observed interatomic distances slightly smaller than the radius sums. It seems doubtful that these deviations are to be attributed to a partially ionic character of the bonds, and the number of other factors which might conceivably be operative is so large that no single one can be selected with confidence as responsible. 

A number of years ago an equation was proposed13 for calculating the partial ionic character of a a bond between two atoms A and B from their electronegativity difference sa — b... 

The charges on the oxygen atoms due to partial ionic character of the bonds to the metal atoms in the silicates and other salts should be taken into consideration in making this calculation. These charges lead to further decreases in the Si-O, P-O, S-O, and Cl-0 distances, of amount depending on the nature of the metal and the structure of the crystals. Because of uncertainties in the system of equations used in this paper, this refinement in the calculation has not been carried out. 

The ratio q/e gives the fraction of an electron that appears to be transferred from one atom to another. This ratio can also be considered as the partial ionic character of the bond between the atoms. It follows that the percent of ionic character is 100 times the fraction of ionic character. Therefore,... 

The catalytic activity of the trityl moiety was unobjectionably adjusted in the addition reaction of the allylstannanes to aldehydes [148]. In this allylation process the trityl chloride 52, due to its disposition to partially ionic character of the halogen bonding, was employed as a catalyst in the complementary tandem with weak Lewis acid TMSCl (Scheme 57). The excess of the silyl component was necessary in order to release the trityl catalyst from the intermediate to complete the catalytic cycle. The achieved yield was 93%, when trityl chloride 52 was used. 

One classical example that apphes the electroneutrality principle is the electronic structure of carbon monoxide, a diatomic molecule with a very small dipole moment of 0.110 debye. The only electronic structure that satisfies the octet rule for CO is C=0 , a structure that corresponds to C and O, if the shared electron pairs are equally devided by the two atoms. Pauling showed that the electronegativity difference of 1.0 would correspond to about 22% partial ionic character for each bond, and to charges of and 0° +. A second possible electronic structure, C=O , does not complete the octet for carbon. The partial ionic character of the bonds corresponds to C0.44+ If these two structures contribute... 

The Partial Ionic Character of Covalent Bonds and the Relative Electronegativity of Atoms... 

The discussion of the amount of partial ionic character of simple bonds will be continued in Section 3-9. 

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