Ionicity index

Fig. 3. Correlations between ionic index (z2//) and complexation constants log10 /3f for (a) the monohydroxo, (b) the monofluorido, and (c) the monosulphato complexes of hard cations and the borderline cation Ni(II).

To eompare eompounds that are not elosely related, we must obtain another kind of energy Ep K) by replaeing in (8) the partial charges qi and qj on atoms i and j by the corresponding oxidation numbers z, and Zj. With this full ionic energy Ep K) we ean define an unambiguous ionicity index Ip K) as follows ... 

If Ip K) greater than 0.5, the structure under investigation should be a rather ionie one whereas lower values may indicate partial covalent character. We prefer to use this ionicity index instead of the Madelung constant,... 

Obviously, this moment does not include all the polarization effects induced by the charge distribution on the lone pairs contributions, and should be taken just as a molecular ionicity index and not as the true molecular dipolar moment. 

TABLE 8. Refinement factor R(%), mean electronegativities e 5 ) (eV) atomic sizes r(M) (pm), Mulliken electronegativities (eV), Ewald summation indexes p, Ewald summation parameters K (A ), Madelung energies M (eV), ionicity indexes / (%), minimum interatomic distances Rmin (pm) and Madelung constants Rmin) of the eleven pyrosilicates displayed in figure 8. ( ) Mg values (KBa) = 198 pm, exo (Ba) = 1.81 eV). 

In their AIM discussion on 120 alkali halide perovskites Luana et aZ. discussed the shape of the ions and showed how the topological description contained the classical picture in terms of slightly deformed spheres. They enthusiastically concluded from this and previous work that AIM supplied a rigorous foundation for important historical concepts like ionicity, index of coordination, coordination polyhedra or atomic/ionic volume in a solid. 

The (X is electronegativity and r is ionic radius) reflects the degree of covalent interactions in the metal-ligand complex relative to ionic interactions (Nieboer and Richardson 1980). In QSAR studies, the covalent index is commonly used in combination with ionic index T lr or with the constant for the first hydrolysis Hog KOHI (see Section 3.5). The subscript m refers to the most common (Mulliken) measure of electronegativity. Sometimes other measures are used in the literature and sometimes the m is omitted, e.g., the covalence index in Chapter 8 does not include the m subscript because the Pauling, Mulliken or Allred-Rochow scales for electronegativity might be pertinent. 

QCARs for Predicting Cation Toxicity and Bioconcentration Factors for the Mussels, Mytilis edulis and Perna viridis Using the Covalent Radius, Logarithm of the First Hydrolysis Constant, Pearson and Mawby Softness Parameter, and the Ionic Index... 

As discussed in Section 5.2.5, Van Kolck et al. (2008) developed 4 QSARs to predict the 96-hour LC50 values of 5 cations to the mussel Mytilis edulis and 4 QSARs to predict the 96-hour LC50 values of 6 cations to the mussel Perna viridis (Table 5.17). Six of these QSARs included 3 of the less numerous physical properties used to predict cation toxicity, viz., covalent index (x r), absolute value of the logarithm of the first hydrolysis constant (Hog XqhI), and ionic index (Z /r). The QSARs developed with the covalent index (x r) produced the highest value (Table 5.20). 

Van Kolck et al. (2008) developed 4 QSARs to predict the bioconcentration factors (BCF) of cations to the mussel Mytilus edulis, and 4 QSARs to predict the BCFs of cations to the mussel Perna viridis (Table 5.4). The BCFs for Mytilus edulis were developed for 8 cations and the QSARs with highest values were obtained using the ionic index (ZVr) and the covalent index (x i) (Table 5.18). The BCFs for Perna viridis were developed for 7 cations and the QSARs with highest values were obtained using the Pearson and Mawby softness parameter (Op) and the covalent index xit) (Table 5.18). 

Regression equations were developed between individual metal ion characteristics and toxicity data (LCj(,s) for two species of mollusks, Pema viridis and Mytilus edulis. The four ion characteristics studied were the covalent index, softness index, hydrolysis constant, and ionic index. 

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