Electrostatic covalent model, qualitative

If a qualitative rationalization of bonding has any basis in fact, at the very least one should be able to fit bond strengths, e.g. adduct formation enthalpies, to empirical parameters that relate to the effects used in the qualitative description. If additional parameters are needed for a good quantitative fit, then an added effect is also needed for the complete qualitative rationalization. The equation proposed (fi S) for the qualitative electrostatic-covalent model is ... 

Within the last 5 years a number of quantum mechanical calculations have been carried out, in particular for hydrogen-bonded systems such as HgO, NHg, and HF as solvents. Contrary to electrostatic models, MO calculations allow for the possibility of covalent bond formation and, consequently, constitute a fundamentally better approach to ion-molecule interactions. Some interesting results of recent model calculations, although qualitative in nature, are discussed in Section V. 

Although the above, and similar, arguments are qualitative, they provide a helpful means of assessing the pattern in solubilities for series of ionic salts we stress ionic because equations 6.55 and 6.56 assume an electrostatic model. Our discussions in Section 5.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

In contrast to the claim (10) that the ECW model "disguises the relationship between reactivity and periodic elemental properties", elementary application of frontier molecular orbital theory (H) can be used to understand the trends. Using qualitative trends in ionization energies, inductive effects, electronegativities and partial charge/size ratios, one can estimate trends in the HOMO-LUMO separation of the donor and acceptor. Increasing the separation decreases the covalent and increases the electrostatic nature of the interaction. Decreasing the separation has the opposite effect. Trends in the reported acid and base parameters as well as in the Ey E0 and C C0 products can be understood in this way. 

Anyone offering or considering a different qualitative model for donor-acceptor reactivity should express it using an empirical equation, as was done with Equation 2 for the covalent-electrostatic model. The 500 enthalpies compiled (9) should be fit to the proposed equation. If the quantitative expression of the model leads to a poor fit but reproduces the correct trends with parameters consistent with the model imposed then it can be concluded that the qualitative model is acceptable. If the quantitative expression does not reproduce the trends, then the qualitative model is not acceptable. If the new model requires... 

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