Ionic-Covalent Parameter

Lenglet, M. Spectroscopic study of the chemical bond in 3d transition metal oxides. Correlation with the ionic-covalent parameter. Trends Chem. Phys. 1997, 6, 121-543. 

Portier, J., Campet, G., Etourneau, J., Shastry, M.C.R., and Tanguy, B. A simple approach to materials design role played by an ionic-covalent parameter based on polarizing power and electronegativity. J. Alloys Compd. 1994, 209, 59-64. 

A number of useful properties of the Group 1 elements (alkali metals) are given in Table 8. They include ionization potentials and electron affinities Pauling, Allred-Rochow and Allen electronegativities ionic, covalent and van der Waals radii v steric parameters and polarizabilities. It should be noted that the ionic radii, ri, are a linear function of the molar volumes, Vm, and the a values. If they are used as parameters, they cannot distinguish between polarizability and ionic size. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

The E and C parameters are consistent with chemical intuition and with the earlier qualitative explanations of donor and acceptor strengths in terms of the ionic-covalent bonding and in some cases with... 

In our original work, we used an ionic-covalent model to interpret the E and C parameters. It has been suggested that our E and C parameters are a quantitative manifestation of the hard-soft model. "Softness (or hardness") can be considered (67) as a measure of the ratio of the tendency of a spedes to undergo covalent interaction to the tendency of the species to undergo electrostatic interaction. The relative "softness or hardness is depicted in the C/E ratio. The ratios for the acids and bases can be calculated from the data in Tables 3 and 4. If the ratio C/E is comparatively large, the add or base would be classified as type B or soft. Inasmuch as the relative ratios of C/E tells the relative importance of the two effects for various donors and acceptors, we agree that the hardness or softness discussed in the HSAB model is given by this ratio. 

The effective ionization sphere has a characteristic value for each element and provides a direct measure of electronegativity, the basic parameter that quantifies all chemical interactions. The relative difference in electronegativity determines the extent and nature of valence-electron redistribution, which in turn differentiates between the major types of interaction, commonly known as ionic, covalent, metallic etc. 

The recently available spectroscopic data and the RKR potentials of the alkali hydrides allow us to determine the "experimental" values of the parameters relevant to the transition probability of the charge transfer processes. In the Landau-Zener model these parameters are the energy gap between the and X S adiabatic potentials at the avoided crossing distance and the coupling matrix elements. In this paper the coupling matrix elements are evaluated in a two-state ionic-covalent interaction model. The systema-tic trends found in the alkali hydride series for their X e potentials are presented. This leads to a simple model for the ionic potentials. 

Table III. Parameters for the ionic-covalent interaction in the two-state approximation.

The values of the TB parameters that reproduce the ab initio band structure are reported in Table 2 for a subset of materials the others show the same behaviour. The variation of Aa and p/Aa as a function of the mixing parameter of the hybrid functional is also depicted in Fig. 6. Trends are again very clear on increasing the fraction of HF exchange, the band gap Aa increases (the change is higher at the HF end of the series) this feature makes the material more ionic. The parameter p/Aa that is the TB definition of covalence in the materials, decreases systematically on moving from the pure DFT to the HF end of the series. 

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... 

Eaq and Caq are the tendency of acid A and base B to undergo ionic and covalent bonding, respectively. Equation (2) resembles that proposed by Drago et al. (18) to model heats of complex formation of acids and bases in solvents of low dielectric constant. Only Lewis acids of ionic radius greater than 1.0 A obey Eq. (2). For all smaller Lewis acids, a third pair of parameters has to be introduced ... 

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