Polarizing power

Predict and explain periodic trends in the polarizability of anions and the polarizing power of cations (Section 2.13). 

Arrange the cations Rb" ", Be2+, and Sr23 in order of increasing polarizing power. Give an explanation of your arrangement. 

Beryllium compounds are very toxic and must be handled with great caution. Their properties are dominated by the highly polarizing character of the Be2+ ion and its small size. The strong polarizing power results in moderately covalent compounds, and its small size limits to four the number of groups that can attach to the ion. These two features together are responsible for the prominence of the... 

Rb+ < Sr2+ < Be2+. Smaller, more highly charged cations have greater polarizing power. 

Whether zinc is a main-group or transition metal depends, of course, on one s definition of transition metal and main-group metal. Those who classify zinc as a main-group metal cite its (almost) exclusive oxidation number of +2 in compounds (but see Section 2.06.15.2) and the absence of a partially filled r/ shell in the metal and its compounds. Those who classify zinc as a transition metal usually note its much greater effective nuclear charge, polarizing power and its limited, but well defined, coordination chemistry. 

Element Atomic Radius (A) Ionic Radius (A) (4- coordination) Electro- negativity Polarizing Power (charge/radius2)... 

The Co(III) center is generally regarded as lacking the polarizing power of a proton but its attachment to the N-terminus of an amino acid ester, as in [Co(NH3)5(GlyOEt)]3, accelerates OH -catalyzed hydrolysis by ca. 100-fold. This is similar to the effect observed for N-protonation. 

In applying these ideas, polarizing power and polarizability must be interpreted in terms of the electronic structures of the central metal and the ligands. Polarizing power is not simply a function of z/r [thus note that the ionizing po-. 

Alcohols are hydroxylated alkyl-compounds (R-OH) which are neutral in reaction due to their unionizable (OH) group (e.g., methanol, ethanol, isopropanol, and w-butanol). The hydroxyl of alcohols can displace water molecules in the primary hydration shell of cations adsorbed onto soil-solid and sediment-solid clay particles. The water molecule displacement depends mainly on the polarizing power of the cation. The other adsorption mechanisms of alcohol hydroxyl groups are through hydrogen bonding and cation-dipole interactions [19,65],... 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

Many nonionic organic contaminants require extreme acid conditions to accept H+ ions. In clays, the extent of protonation is related to the electronegativity and polarizing power of structural metal cations, in the order H+ > Al + > Fe > Mg +... 

The simplest approach to describing the interactions of metal cations dissolved in water with solvent molecules is the Born electrostatic model, which expresses solvation energy as a function of the dielectric constant of the solvent and, through transformation constants, of the ratio between the squared charge of the metal cation and its effective radius. This ratio, which is called the polarizing power of the cation (cf Millero, 1977), defines the strength of the electrostatic interaction in a solvation-hydrolysis process of the type... 

Figure 8.14 Effect of polarizing power on degree of hydrolysis of cations in water. Reprinted from D. R. Turner, M. Whitfield, and A. G. Dickson, Geochimica et Cosmochimica Acta, 45, 855-881, copyright 1981, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Figure 8.16 shows how the complexing constants of ions vary with the polarizing power of the cation, ligand being equal. Note also here that the abscissa... 

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