Ionic contribution to bonds

Calculated assuming covalent and ionic contributions to bonding are in the ratio l/X of the charge derived from dipole moments (152). 

The truth lies somewhere between these two extremes, and the assessment of covalent and ionic contributions to bonding has attracted considerable attention. Pauling devised the electroneutrality principle and suggested that the relative importance of ionic and covalent components was such that the overall charge on any one atom did not exceed 1. In the context of the [Cr(H20)6]3+ ion, this leads to a description in which the metal centre is neutral and each oxygen bears a charge of + 2. The Cr-0 bonds may be described as 50 % covalent. 

Other boron linking patterns found in borides are summarized in Table 6.23. Several bonding theories have been developed, starting from the recognizable networks of boron found in these compounds. For example, NMR and conductivity measurements on diborides suggest that the boron network has a tr bonding component which could be achieved if some electron transfer from metal to boron took place. The model would then be with an ionic contribution to bonding. The extra electron... 

The Dg were shown to increase from the 4d to the 5d compounds and decrease from the 5d to the 6d element compounds for almost all types of species except of group-8 MO4. In [160-168], such a decrease in Dg of the 6d element compounds was explained by a decrease in the ionic contribution to bonding, while the covalent one steadily increases in the groups. Dg obtained as a sum of the ionic and covalent contributions to chemical bonding calculated within the DS DV scheme turned out to be rather accurate [160-167]. 

Macroscopically brittle at room temperature ionic contribution to bonding (charge transfer) deduced from photoelectron spectroscopy and interpretation of heats of formation the stability of these phases (high melting points, for example) attributed to the ionic and /-orbital contributions 111 < 110) and (110 <001> slip systems operative at elevated temperatures... 

Using die values of electronegativity, Pauling further suggested that die fractional ionic contribution to the bond energy (FIG) is related to the electronegativities jca and xq in the A-B molecule and given by... 

The charges on the atoms in HCI are called partial charges. We show the partial charges on the atoms by writing 8+l I—Cl8. A bond in which ionic contributions to the resonance result in partial charges is called a polar covalent bond. All bonds between atoms of different elements are polar to some extent. The bonds in homonuclear (same element) diatomic molecules and ions are nonpolar. 

Note that the bond order index defined by Mayer accounts for the covalent contribution to the bond (this is why of late it is often mentioned as shared electron pair density index, SEDI). As such, the index cannot be expected to produce the integer values corresponding to the Lewis picture if a bond has a significant ionic contribution. The bond order index defined in this way measures the degree of correlation of the fluctuation of electron densities on the two atoms in question [7]. 

The number, 0.75 vu, is sometimes called the formal charge on the O atoms, but it should not be confused with either the formal ionic charge (=-2 for all ions) or the charges on the O atoms calculated by quantum mechanics. Quantum mechanical charges are usually larger than -0,75 (depending on how the calculation is performed) since they include ionic contributions to the P-O bonds as well as to the external bonds. Quantum mechanics does not allow one to separate the internal and external bond contributions. 

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