Lewis acid-base character

LEWIS ACID-BASE CHARACTER AND STABILITY OF ANION AND CATION RADICALS. 

Recently, Grochala and Peters have shown that the thermal decomposition temperature of many binary hydrides could be correlated with the standard redox potential for the metal cation/metal redox couple and with the standard enthalpy of decomposition [18, 83[. They also showed that, for multinary hydrides, the decomposition temperature could be tuned by a careful choice of the stoichiometric ratio and of the Lewis acid/base character of the constituent elements. 

Modification of the Lewis acid-base character of a. surface can also be achieved by the presence of inorganic ions in the scdution. In this case, however, a net correlation between the (more representative) value of Ys the entropy of hydration of the ions. 45i, has been found (48). In Fig. 9 a linear correlation is observed between those quantities for a series of cations and anions, with the only exception of K and OH which show a special behavior. Although AS deals with ions in solution and describes the surface behavi[Pg.148]

Surface treatments for fillers have been extensively reviewed [124]. Ernstsson and Larsson studied a range of mineral fillers in terms of the Lewis acid-base character [125, 126]. It was found that each one had a different surface chemistry and they were all amphoteric (possessing both Lewis acid and basic sites). Another point was that trace impurities of iron on the filler surface, presumably picked up during processing, significantly changed the surface chemistry of silica. 

Figure 6.11 illustrates the Lewis acid-base character of the solid surfaces presented in Fig. 6.10. The Lewis acid hA and Lewis base components are related to the...

Most organic reactions are Lewis acid/base processes that involve the interaction of a nucleophilic center with an electrophilic center. Because electrochemistry provides the ultimate nucleophile via the electrons at the cathode surface and the ultimate electrophile via the electron holes at the anode surface, it is the ideal methodology for the characterization of the electrophilicity and nucleophilic-ity of molecules. Thus, the carbon centers of saturated hydrocarbons (e.g., CH4) are resistant to electrochemical reduction and oxidation because of their inert nature (all valence electrons are stabilized in sigma bonds an absence of any Lewis acid/base character). However, organic molecules with electrophilic components [e.g., alkyl-, aryl-, and acyl- halides carbonyl groups unsaturated and aromatic hydrocarbons nitro groups Brpnsted... 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

The hard-soft acid-base principle is not restricted to the usual types of Lewis acid-base interactions. It is a guiding principle for all types of interactions that species of similar electronic character interact best. Accordingly, we will refer to the principle as the hard-soft interaction... 

Cationic zirconocene complexes are well-known very reactive species with interesting applications owing to their Lewis acidic character see Lewis Acids Bases). In particular, they are often involved into various catalytic processes including Ziegler-Natta type polymerizations see Ziegler-Natta Catalysts). 

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