Metal ions polarizability

The alkali-metal-ion polarizability increases from Li+ to Cs" ". Thus, the binding energy of any ligand L in the MLn" complexes decreases with increasing alkali metal ion size Li > Na+ > K" " > Rb > Cs. ... 

If the concentration of the metal ion is not negligible at the potential of zero charge, the electrode potential varies linearly with log c according to Eq. (2) and there is no distinctive sign of the situation where the charge at the interface vanishes. The Nemst approach is obviously unsuitable for defining the nature and the amount of the charge at an interface. If the concentration of the metal ion at the pzc is small or very small, the behavior of the interface becomes that of a polarizable electrode. 

In addition to the interactions discussed above, which all depend in part on the ioniz-ability, or at least polarizability, of the surface and the adsorbates, hydrophobic parts of ligands may bind to corresponding parts of surfaces. Thus, if a metal ion is complexed or irreversibly bonded to a hydrophobic molecule, the metal may be incorporated into the bulk or surface of a particle via hydrophobic interaction between the molecule and the solid phase. Such interactions may be quantitatively significant in systems with high concentrations of dissolved and particulate organic matter. 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

A bar of talc feels like a bar of soap which is why it is often called soapstone. Its exceptional softness (it is the softest of the Mohs minerals) is a direct result of its unusual crystal structure. This consists of sheets of silicate tetrahedra without metal ions between the sheets. Thus the sheets are bonded only by London polarization forces. The latter are particularly weak because silicate tetrahedra have relatively small polarizabilities. 

It is also interesting to note that metal ions having low polarizability (Al3+ Be2+ etc.) are those that are acidic (as shown in Eq. (9.17)). Also, in Chapter 7 we discussed how the polarization of ions leads to a lattice energy that is higher than that predicted on the basis of electrostatic interactions alone. The polarizability data shown in the table make it easy to see that certain ions are much more polarizable than others. Although we will not visit again all of the ramifications of electronic polarizability, it is a very useful and important property of molecules and ions that relates to both chemical and physical behavior. 

The interface is in contact with two bulk phases, the metal electrode (index m ) and the solution (index s). Formally, we consider the metal to be composed of metal atoms M, metal ions Mz+, and electrons e " these particles are present both in the electrode and the interface, but not in the solution. On the other hand, certain cations and anions and neutral species occur both in the solution and the interface. Since the electrode is ideally polarizable, no charged species can pass through the interface. 

In terms of a ligand s polarizability or covalent donor ability, the nephelauxetic series is of use for comparative purposes (318). A series 0-Me2S0 0-(CH2)4S0 > H20 > C5HsNO has been presented (163), but other data (101) suggest that 0-Me2SO be placed between the N-SCN- and CN- ions. A generally accepted series compiled from many results (319) is I- > Br > CN- Cl- > IV-NCS- > [oxalate]2- en > NH3 > urea > H20 > F-. As this series is approximately independent of the metal ion in the complex, some discrepancy is apparent. 

The charge delocalization or the polarizability difference explains the selectivity behaviour in cases of high polarizability differences (complex versus aqueous metal ion) or in a homologous series of ions (either inorganic cations or ammonium cations). The smaller hydration status of all types of interlamellarly adsorbed cations is ascribed to the mutual stabilization by charge delocalization over the planar oxygens and exchangeable cations and is caused by the electrostatic interaction forces. 

Acceptors may be considered as either hard"or soft". Hard acceptors, such as the proton or alkali metal ions are hardly polarizable and tend to react preferentially with light donor atoms14, ls ... 

The rotational barrier increases with increasing ionic radius of the metal ion. We have noted above that rj is highly linear in the polarizability parameters a and Vm and are therefore unable to determine whether this dependence is due to ionic size or ionic polarizability. 

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