Character in Predominantly Ionic Bonds

4 Bending Models In Inorganic Chomiitryi I Ionic Compounds 

Fajans suggested the following rules to estimate the extent to which a cation could polarize an anion and thus induce covalent character. Polarization will be increased by  

Te and highly charged ones such as As and P are especially prone to polarization and covalent character. 

A second area in which polarization effects show up is the solubility of salts in polar solvents such as water. For example, consider the silver halides, in which we have a polarizing cation and increasingly polarizable anions. Silver fluoride, which is quite ionic, is soluble in water, but the less ionic silver chloride is soluble only with the inducement of complexing ammonia. Silver bromide is only slightly soluble and silver iodide is insoluble even with the addition of ammonia. Increasing covalency from fluoride to iodide is expected and decreased solubility in water is observed. 

Silver fluoride Silver chloride Silver bromide Silver iodide 

Fajans considered the effect which a small, highly charged cation would have on an anion. If the anion were large and soft enough, the cation should be capable of polarizing it, and the extreme of this situation would be the cation actually penetrating the anionic electron cloud giving a covalent (shared electron) bond (Fig. 4.19). 

Fajans, K. Natunvissenschaften 1923. //. 165. For a more recent discussion of the same subject, see Fajans. K. Struct. Bonding Berlin 1967, J. 88-105. For an interesting short sketch on the theory and the man. see Hurwc. J. J. Chem. Educ. (987.64, 122. 

4 Bonding Models In Inorganic Chemistry I. Ionic Compounds 

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From the preceding, it might be supposed that covalent character in predominantly ionic compounds always destabilizes the compound. This is not so. Instability results from polarization of the anion causing it to split into a more stable compound (in the above cases the oxides) with the release of gaseous acidic anhydrides. As will be seen in Chapter 16, many very stable, very hard minerals have covalent-ionic bonding. 

It follows from this brief introduction that in order to understand the subtleties in melting point trends, one needs to somewhat quantify the extent of covalency present in an ionic bond. In Chap. 2, the bonds between ions were assumed to be either predominantly covalent or ionic. As noted then, and reiterated here, the reality of the situation is more complex — ionic bonds possess covalent character and vice versa. Historically, this complication has been addressed by means of one of two approaches. The... 

Experiments with 90 percent enrichment in the 1 or 2 position of 1,3 butadiene confirm earlier work that the live end is predominantly a 1,4 unit with a trans/cis ratio of /l The Li is bound to the Q( carbon of the 1,4 butadiene unit in what appears to be a highly localized 0 bond However, the presence of partial ionic character in the bond cannot be ruled out There is no evidence of Li being gf bonded to the carbon When a chelating diamine such as dipiperidyl ethane is added to the live cement, a drastic change takes place in the spectrum which suggests complete conversion to a delocalized ionic bonding. 

Because the 4f orbitals are deeply buried, they play little part in metal-ligand bonding, which is predominantly ionic in character. Subsequently, coordination numbers and geometries are mostly controlled by spatial requirements of ligands. Coordination numbers of 8 or 9 are most common, but have been reported up to 12 (34,35). 

The organometallic compound chemistry of the 2A metals is similar to that of the 1A metals, and ionically bonded compounds predominate. As is the case with lithium in group 1 A, the first 2A element, beryllium, behaves atypically, with a greater covalent character in its metal-carbon bonds. 

This approach, which is certainly not exact, breaks down badly if extended to predominantly covalent bonds. To evaluate the degree of ionic character in such bonds if they occur in simple molecules, Pauling has used dipole moments. The term dipole has already been applied to the water molecule, a molecule in which there is a separation between the centers of positive and negative charge (Chap. 4). When the charges are equal in magnitude, the dipole moment, ju, is defined... 

Although molecules are held together by bonds that are predominantly covalent, many substances are made up of ions that are arranged in a crystal lattice. These materials are held together in the solid state by forces that are essentially electrostatic in character. In some cases, the forces arise from the transfer of electrons between atoms to produce ionic materials. However, in most cases the ions are somewhat polarizable (especially anions), so the ions have distorted structures that represent some degree of electron sharing. As a result, many of the forces in crystals that are normally considered to be ionic may be appreciably less than completely ionic. This fact should be kept in mind as the principles of ionic bonding are discussed. 

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