Covalent-ionic resonance

Pauling later expressed the view that partial covalency or ionic-covalent-resonance contributions of the form... 

This shift from polarization to ionic-covalent resonance led to following contradictions in which CsF played a prominent role. 

The relative magnitudes of a and b will determine whether the bonding is better described as ionic or covalent. If the energy Ec associated with tp (covalent) is lower than Eh the energy associated with (ionic), a will be greater than b, and vice versa ionic-covalent resonance ensures that the true energy E of the system will be lower than either Ec or E,. However, it seems that in many cases Ec and E, are not very different and either a covalent or an ionic description affords a reasonable approximation. 

In SiF4 n bonding, using F 2p orbitals and Si 3d orbitals, could contribute to the shortening. It is probable that both ionic-covalent resonance and multiple bonding contribute significantly. 

Similarly, in SiF4, even after due allowance for the effect of ionic-covalent resonance in strengthening and hence shortening the Si—F bonds, they appear to be around 0.13 A shorter than the length expected for single bonds, and the implication, therefore, is that n bonding is present as well. 

Strengthening and hence shortening of the B—X bonds by ionic-covalent resonance, especially for B—F and B—O bonds because of the large electronegativity differences. Evidence that this is important, in addition... 

Presumably this reflects an increasing stabilization of the saturated product of the reaction, with greater fluorine content, due to ionic-covalent resonance. 

The explanation for this increase probably lies in an increase in ionic-covalent resonance stabilization in the sequence TiCl ... 

The MO function (13.108) underestimates electron correlation, in that it says that structures with both electrons on the same atom are just as likely as structures with each electron on a different atom. The VB function (13.109) overestimates electron correlation, in that it has no contribution from structures with both electrons on the same atom. In MO theory, electron correlation can be introduced by configuration interaction. In VB theory, electron correlation is reduced by ionic-covalent resonance. The simple VB method is more reliable at large R than the simple MO method, since the latter predicts the wrong dissociation products. 

TTie optimum values of and C2 are found by the variation method this leads to the usual secular equation. We have ionic-covalent resonance, involving the structures H—F and H F . Tlie true molecular structure is intermediate between the covalent and ijonic structures. A term CsIIshI h corresponding to the ionic structure H"F could also Ije included in the wave function, but this should contribute only slightly for HE For molecules that are less ionic, both ionic structures might well be included. 

Shaik S, Maitre P, Sini G, Hiberty PC (1992) The charge-shift bonding concept Electron-parr bonds with very large ionic-covalent resonance energies. J Am Chem Soc 114(20) 7861-7866... 

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