Partial Ionic Character of Multiple Bonds

THE EFFECT OF UNSHARED PAIRS ON BOND ENERGIES AND BOND LENGTHS94 

The bond energies of the bonds N—N, O—O, and F—F show a striking abnormality their values are much smaller than would be expected from extrapolation of the corresponding values for their congeners, whereas the values for I -i—Li and C—C (and also for Be—Be and 

not gi /en in Chapter 3) are about as expected (Sec. 3-5). The bond lengths of these bonds are also larger than would be expected in comparison with those of their congeners (Chap. 7). These abnormalities are to be attributed to the strong repulsion of the unshared electron pairs of the bonded atoms (and of the electrons involved in the other bonds in the case of N—N and 0 0). 

The quantum-mechanical theory of valence leads to the result that the interaction energy of two unshared electron pairs on adjacent 

For F—F, for example, the bond energy 100 keal/mole would be expected from extrapolation of the sequence I—I, Br —Br, Cl—Cl, with Sn—Sn, Ge--Ge, Si—Si, C—C as a guide. This value is 63 kcal/mole greater than the actual value, 36.6 kcal/mole. It therefore corresponds to a value of the energy of repulsion of the unshared electron 

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