Bond ionic character

Fig. 3. Dependence of the metal-halogen bond ionic character on the difference in the metal and halogen electronegativities (values of the electronegativities were obtained from references (65), (66) and (67))...

The calculation of the bond ionic character in the above way is, of course, inaccurate. But in spite of all these limitations, the simple approach described... 

Bond Ionic character (%) Bond Ionic character (%)... 

The calculation results of dissociation energy by the Eq. (4.7), given ion Table 4.2, demonstrated that Pc=flo- some molecules containing such elements as F, N and 0, the values of ion radius have been applied to register the bond ionic character for calculating Pg-parameter (in Table 4.2 marked with ). For such molecules as C, N, the calculations have been made by multiple bonds. In other cases the average values of bond energy have been applied. The calculated data do not contradict the experimental ones [2, 3]. 

On the basis of the above interpretation NQI can be unequivocally represented in terms of the population of orbitals. Actually, NQR spectroscopy allows the determination, at best, of two parameters (e Qqh and rj) and in many cases (I = j) only e Qqh can be obtained. However, the above-mentioned Equation [1] contains four parameters (s, d, /g, which cannot be determined from one or two experimental parameters. It is therefore necessary to include approximations, which neglect the d orbital participation in hybridization, and also, in some cases, p bonding and to consider that the s orbital hybridization is a small contribution and remains constant in a series of compounds. Thus changes of e Qqh are directly related to bond ionic character or p electron charge transfer in the case of a hydrogen bond. 

There is a great number of mostly covalent and tetraliedral binary IV-IV, III-V, II-VI and I-VII semiconductors. Most crystallize in tire zincblende stmcture, but some prefer tire wairtzite stmcture, notably GaN [H, 12]. Wlrile tire bonding in all of tliese compounds (and tlieir alloys) is mostly covalent, some ionic character is always present because of tire difference in electron affinity of tire constituent atoms. 

When the several halides of a given element are considered, changes in bond character are also found. The fluoride is generally the most ionic with ionic character decreasing from fluoride to... 

The additional terms in the molecular orbital wavefunction correspond to states with the two electrons in the same orbital, which endows ionic character to the bond The... 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

The greater the difference in electronegativity, the greater is the ionic character of the bond. The amount of ionic character I is given by ... 

The participation of phosphoms d orbitals in the five- and six-coordinate compounds provides increased polarizabiUty, nucleophilicity, and ionic character. In fact, compounds such as phosphoms pentachloride [10026-13-8] 5) thought to have considerable ionic character. The CJ-bond orders between the phosphoms atom and its constituents in the higher coordinate compounds maybe less than one. 

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

FIGURE 18.28 The structure of cyanocobalamin (top) and simplified structures showing several coenzyme forms of vitamin Bi2- The Co—C bond of 5 -deoxyadenosylcobalamin is predominantly covalent (note the short bond length of 0.205 nm) but with some ionic character. Note that the convention of writing the cobalt atom as Co" " attributes the electrons of the Co—C and Co—N bonds to carbon and nitrogen, respectively. 

In conclusion it appears that the indole magnesium halides are essentially N—MgX species, with the degree of ionic character of the N—MgX bond being markedly affected by external factors, such as... 

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