Fajans covalency rules

Basicity of Ions Due to lanthanide contraction from La3+ to Lu3+ covalent character increases between Lanthanide(III) ions and OH- ions. Thus La(OH)3 is most basic while Lu(OH)3 is least basic. According to Fajan s rule (Basicity decreases as ion size decreases). These differences in basicity are reflected in... 

Since the type of binding has such an influence upon the physical and chemical properties, rules to help us to anticipate whether a given compound will be ionic or covalent will be extremely useful. Such rules, though only qualitative, are extremely valuable in helping to correlate the properties of elements with their valence numbers and position in the periodic table. There are four covalency rules, the first three being due to Fajans and the fourth to Sidgwick. 

Charge density gives a measure of how much and how tightly packed the charge is around an atom. According to Fajan s rule, the charge density of a cation affects the extent of covalent bonding. See Chapter 11 for more details... 

Fajan s rules dictate that the partial covalent nature of a metal s ionic bond with an anion will increase with high charge of either of the interacting ions, small cation radius, or large anion radius. The covalent nature of the bond also is higher with a noninert gas electron configuration of the cation, e.g., the more covalent nature of Cu + ( Ar 3d ) bonds, in contrast to the ionic bonds formed by Na+ ( Ne ) (Barrett 2002). 

Fajans rules state that the extent of polarization of an anion or the partial covalent character of an otherwise 100% ionic compound is increased by ... 

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... 

These reactions illustrate the general rules described by Fajans in 1923. They can be summarized in four rules, in which increased covalent character means lower solubility, increased color, and shorter interionic distances ... 

The latter approach was championed by Fajans and is embodied in Fajans rules, whose basic premise is summarized in Fig. 4.2. In Fig. 4.2a an idealized ion pair is shown for which the covalent character is nonexistent (i.e., the ions are assumed to be hard spheres). In Fig. 4.2 > some covalent character is imparted by shifting the electron cloud of the more polarizable anion toward the polarizing cation. In the extreme case that the cation is totally embedded in the electron cloud of the anion (Fig. 4.2c) a strong covalent bond is formed. The extent to which the electron cloud is distorted and shared between the two ions is thus a measure of the covalent character of that bond. The covalent character thus defined depends on three factors ... 

The result is that the first element of the main groups (lithium, beryllium as well as boron, carbon, nitrogen, oxygen, and fluorine described in Chapter 12) are different from the rest of their group members. Usually, the first element is much smaller than the rest of the elements in that group, and it tends to form covalent compounds and complexes. This tendency is called Fajans rule after Kazimierz Fajans, a physical chemist who in 1923 postulated it. 

Fajans rules Rules indicating the extent to which an ionic bond has covdent character caused by polarization of the ions. Covalent character is more likely if ... 

In 1923, Kasimir Fajans established a qualitative set of rules (known as Fajans rules) that allows one to predict the relative degree of covalent character in this intermediate arena of bonding. The covalency of the bond will increase with the degree of polarization. This, in turn, will increase with (i) cations having large charge densities, (ii) anions having diffuse electron clouds, and (iii) cations that have partially filled d orbitals. 

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