Percentage ionic character

FIGURE 2.13 The dependence of the percentage ionic character of the bond on the difference in electronegativity, A, between two bonded atoms for a number of halides. 

Table 3.31. Trigonal bipyramidal anions, comparing central-atom valency Vm (and percentage ionic character), d-orbital occupancy du, bond lengths Rmx, bond orders />mx (andpercentage ionic character), and ligand atomic charges Qx for oo-bonded (SiH3 , SiFs-, and SiH3F2 ) versus non-w-bonded (CH3F2-)...

Table 3.33. NBO/NRT descriptors of molecules in Table 3.32, including atomic charges (Q), central atom d-orbital occupancy (c/m )> NRT bond orders, and central-atom valency (with percentage ionic character)...

Table 3.35. Natural atomic charges (Q), d-orbital occupancies (d f), and NRT bond orders and valencies (with percentage ionic character) for ArF species...

See Pauling, note 14, pp. 97ff. Coulson, note 21, p. 141, states Now there must be some relation between the percentage ionic character and the electronegativity difference xB — xA. It is of the very essence of the idea of an electronegativity scale that this should be so. ... 

The HC1 molecule has a dipole moment of 1.03 D and a bond length of 128 pm. Calculate the percentage ionic character, and compare your answer with that for HF. Comment on the differences in electronegativity coefficients in the two molecules. 

For a diatomic molecule XY, Molecule = Covalent + 0.50i//lonic, calculate the percentage ionic character of the X-Y bond. If the X-Y bond length is 150 pm, what is the approximate dipole moment of XY ... 

The Townes and Dailey method thus describes chemical bonding in terms of three parameters ionic character, s-hybridization and multiple bonding, whilst at the most two parameters are experimentally available, ezQqjh and rj. Even in such favourable cases where the extent of multiple bonding can be shown to be zero if it is axially symmetric or determined from the asymmetry parameter if it is not, one is still left with the problem of deciding the extent of either s-hybridization or the percentage ionic character. 

The physical and chemical properties of the inorganic azides have been extensively reviewed [4-11], Richter [12] has discussed the chemical classification of azides as (i) stable ionic azides, (ii) heavy-metal azides and (iii) unstable covalent azides. This classification is based on the percentage ionic character of the metal-azide bond, tabulated as formal ionicities in [12]. For example, the Na-Nj and Ba-Nj bonds are 70% ionic, but Pb-N, is only 34% and H-Nj is 22%. Bertrand et al. [13] have reviewed the photochemical and thermal behaviour of organometallic azides. Richter has also given [12] an excellent review of the methods of preparation of HNj and other azides. He criticizes early workers for inadequate purification and characterization of their starting materials and their neglect of allowance for the possible formation of hydrates (e.g., barium azide may be present as Ba(N3)2.1. SHjO below 284 K, forms a monohydrate between 284 and 325.5 K and is anhydrous above 325.5 K). 

Table 1 Partial list of electronegativities and percentage ionic character of bonds with oxygen.

The percentage ionic character in the bond is related to X by equation 1.26 ... 

There have been attempts to relate differences in electronegativity coefficients to the percentage ionic character of bonds between different elements. These are reasonable on a qualitative level, but quantitative equations are found only to apply roughly to particular series of compounds e.g. H-F to H-1), no broader generalizations being reasonable. 

The stability of metal tetrahydroborides has been discussed in relation to their percentage ionic character, and those compoxmds with less ionic character than diborane are expected to be highly unstable [30]. Steric effects have also been suggested to be important in some compounds [31, 32]. The special feature exhibited by the covalent metal hydroborides is that the hydroboride group is bonded to the metal atom... 

Other, currently more specialist but of potential wide applicability, methods include the optical detection of quadrupole resonances—a sample is laser-excited to an electronically excited state, the return to the ground state is by phosphorescence the intensity of the phosphorescence is sensitive to whether or not concurrent microwave radiation matches an energy separation in some quadrupole-split intermediate state. Yet another method depends on correlations between successive p or y emissions from excited quadrupolar nuclei (where the excitation can be achieved by suitable nuclear bombardment). These do not exhaust the list of current developments—they have been chosen to illustrate the wide front on which new techniques are emerging. It is likely that because of these developments the future will see a wider use of NQR spectroscopy. It is also likely that the interpretation of the data will become more sophisticated. Traditionally, the experimental data have been interpreted to give the percentage ionic character of a bond. This is because, for example, in the CP ion all of the p orbitals are equally occupied whilst in CI2 the a bond, if composed of p orbitals only, corresponds to one electron in the p orbital of each chlorine atom, and so CP and Cl 2 differ in their resonant frequencies. Interpolation allows a value for the ionic character of a Cl-M bond to be determined from the chlorine resonance... 

To decide which bond is more polar, look up EN values for H, Cl, and O in Figure 10-6, and then compute electronegativity differences, AEN, for H—Cl and H—O bonds. The greater the electronegativity difference, the more polar the bond. To determine the percentage ionic character, we use the curve in Figure 10-7. 

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