Resonance extra ionic

However, this uninhibited resonance, involving the conversion of atoms into ions (or the transfer of ionic charges to atoms), requires that the atom receiving a bond have an orbital available for its reception. It is the possession of this extra orbital (the metallic orbital), in addition to the orbitals required on the average for occupancy by unshared... 

Pauling calls the extra DE possessed by a molecule with partial ionic character the ionic resonance energy or A. Thus we have the equation... 

In Table 13.4, we illustrate nicely with the HL-P method the need of numerous structures in order to obtain a good description of the resonance, that is, a high value of T. When only the two Kekule structures are considered (case 1), the HL-P trust parameter is rather low (77 %). When this set is augmented with the three Dewar structures (case 2), t slightly increases to 80 %, which is obviously still not satisfactory. One has to add six equivalent para ionic structures to gain almost 10 % from the initial Kekule set (case 3), and another 24 extra meta ionic/covalent contributors to reach 97 % of the spanned space (case 4). The resulting 35-structure set corresponds to 145 among the 175 VB structures. The major importance of the Kekule... 

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