Ionic limit

Table 9. Ionic radii and ionic limiting molar conductivities of some anions in PC at 25°C, taken from Ref. 1211]...

Polar bonds range from bonds between atoms that have large but slightly less than integral charges and are therefore close to the ionic limit to pure covalent bonds between... 

The definition (3.60) can even be extended to the lone-pair case, in which the ligand is nonexistent, by recognizing that this corresponds to the ionicity limit / = — 1. In this case Eq. (3.60) gives for the formal lone-pair electronegativity ... 

Similarly, the hybridization A.+ of an empty (ionized) hybrid corresponds to the ionicity limit / = +1, which can be formally associated with a ghost (gh) ligand of infinite electronegativity... 

For example, atoms of both the alkaline-earth family (ZAval = 2) and the chalcogen family (ZAval = 6) correspond to FAemp = 2, and their stoichiometric proportionality (or coordination number) to monovalent atoms is therefore commonly two (AH2, ALi2, AF2, etc.). It is a remarkable and characteristic feature of chemical periodicity that the empirical valency FAemp applies both to covalent and to ionic limits of bonding, so that, e.g., the monovalency of lithium (Vuemp = 1) correctly predicts the stoichiometry and coordination number of covalent (e.g., Li2), polar covalent (e.g., LiH), and extreme ionic (e.g., LiF) molecules. Following Musher,132 we can therefore describe hypervalency as referring to cases in which the apparent valency FA exceeds the normal empirical valency (3.184),... 

According to these fractional weightings, the percentage d character of each Os—H bond hybrid is therefore expected to vary between 75 % (for sd3 hybrids, the covalent limit) and 66.7% (for sd2 hybrids, the ionic limit) with changing electronegativity of X ... 

This term is meant to convey relative charge on Cm rather than truly ionic limiting character. Thus, as shown in Table 4.28, Cm is more negative by 0.734 charge units than the adjacent Cp. 

The zero approximate operator Vo projecting on the ionic limit ground state corresponds to u = 1 z = v = 0. To ensure the correct (ionic) limit of a general one-dimensional projection operator in a three-dimensional space, we apply the prescription of eqs. (1.107) and (1.104) with the notion that dim Im Vo = 1 and... 

To conclude this section, we notice that the described discrepancy between the molecular geometry at the oxygen donor atom and the shape of its hybridization tetrahedron is characteristic only for the ionic limit of the dative bond. If the additional... 

These R3TeX compounds may have either tetrameric or dimeric structures, as shown in Fig. 12-12. The Te—X—Te distances are quite long and are to be regarded as secondary, essentially ionic interactions. In compounds like Me3Te+BF4 the ionic limit is approached even more closely. 

The bond flux can also be calculated by recognizing that in the ionic limit each bond is an electric capacitor (represented by flux linking two opposite charges). The network of atoms and bonds is thus a capacitive electrical circuit, and since in most equilibrium structures aU the bond capacitances are empirically found to be equal, the fluxes can be calculated using the Kirchhoff equations (2) and (3) in which the bond capacitances cancel. ... 

A limiting ionic structure with a doubly charged positive chromium ion and six CN ions is not in agreement with the magnetic data, since the Gr + ion has four unpaired electrons. Furthermore, such a structure cannot resonate with the form XXVII since resonance can only occur when the value of the total spin is identical in both forms. The ionic limiting structure must, therefore, be given by XXVIII, An example of an intermediate resonance structure is given by XXIX. 

As expected AoiC, goes to zero in the fully ionic limit (zero resonance integrals / OiA,Cj/j) increases when the degree of covalency j3o,x Cjfi/(eCj ... 

Since charge compensation requires a modification of the charge density, changes of covalency at the surface are often assumed to heal the polarity. With the help of the bond transfer model, one can show that this statement is incorrect, as far as semi-infinite polar surfaces are concerned. It is useful to make a distinction between weakly polar surfaces, in which the dipole moment in the repeat unit is entirely due to covalent effects, and truely polar surfaces whose dipole moment contains an integer contribution. As already said, in the fully ionic limit, the first ones are considered as nonpolar, while the second ones are recognized as polar. 

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