Ionic resonance energy

It is desirable to examine in greater detail the reasons for the thermodynamic instability (small dissociation energy) of the alkyl carbon-transition metal a bond, which appears to be so much less than the carbon-metal a bond of the nontransition metals. The reasons for the instability are (a) the very small covalent energy of the metal-carbon bond and (6) the relatively small difference in electronegativities between the trairsition metal and the carbon atom, which accounts for the small ionic resonance energy contribution to the total energy of the bond. 

One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (-3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, Dq (uncorrected for zero-point vibrational energy), between two atoms i and j, are the covalent bond energy, Xjj, and the ionic resonance energy, IRE. The evaluation of Ay takes the form ... 

The energies are usually expressed as electron volts. The IRE for the bond in ethane is zero and for CHgNa it is 2.56 ev. The stability of alkyl carbon-metal bonds for a variety of metals has been evaluated by Jaffe and Doak (5). They point out that not only is the (the measure of covalent energy) for the C—M bonds of transition metals appreciably smaller (perhaps one-half) than the corresponding values for other elements, but the ionic resonance energy of the alkyl-transition metal bonds is also appreciably smaller (perhaps one-third) than that of alkyl-alkali or alkyl-alkaline earth metal bonds. 

The ionic resonance energy is ihe difference between Ihe experimental bond energy of CIF, 255 kJ mol-1, and the calculated value, 200 kJ mol", or 55 kJ moJ-1 Pauling defined the difference in electronegativity between chlorine and fiuorine as the square root of the ionic resonance energy.45... 

In later chapters you will find examples of the stabilization of covalent bonds through ionic resonance energy. For now. show its importance by predicting whether the molecules NX3... 

Jprgensen57 has referred to this tendency of fluoride ions to favor further coordination by a fourth fluoride (the same is true for hydrides) as symbiosis." Although other factors can work to oppose the symbiotic tendency, it has widespread effect in inorganic chemistry and helps to explain the tendency for compounds to be symmetrically substituted rather than to have mixed substituents. We have seen (Chapter 5) that the electrostatic stabilization of C—F bonds (ionic resonance energy) will be maximized in CF4, and similar arguments can be made for maximizing hard-hard or soft-soft interactions. 

Exacily the same result is obtained if the initial electronegativity of the central halogen is assumed to be higher in a higher oxidation state, and Ax and ionic resonance energy are lower. The same arguments apply equally well to all of the oxidation slates ... 

With such a large energy gap between the VB structures, we can justifiably use perturbation theory to construct the states, and predict the stabilization energy of the ground state by the covalent—ionic resonance energy. This... 

In the general case, the resonance energy for a molecule is defined as the difference between the energy of the ground state and the energy of its most stable VB structure (the reference structure). In the F2 case, the covalent—ionic resonance energy is defined as in Equation 10.10 ... 

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