Hard-acid nature

As this chapter has shown, the oxophilic (hard acid) nature and high- and variable-coordination requirements of the rare-earth cations can frequently be satisfied by... 

Treatment of a typical CA inhibitor, acetazolamide with la. yields the product as a precipitate, which was identified as a I complex 9. wherein the sulfonamide is deprotonated to coordinate with zinc(II) ions. It is o/ interest to point out that with other transition metal ions (e.g., Ni " ), acetazolamide binds to the thiazole N but not to the deprotonated sulfonamide nitrogen. This different behavior of zinc(II) best illustrates its outstanding (hard) acid nature that favors the anionic N donor over the neutral (soft) N donor. The value of 7.4 for... 

The thermodynamic tendency of a substance to act as a Lewis acid. The strength of a Lewis acid depends on the nature of the base with which the Lewis acid forms a Lewis adduct. Hence, comparative measures of Lewis acidities are given by equilibrium constants for the formation of the adducts by a common reference base. See Lewis Acid Electrophilicity Hard Acids Soft Acids Acceptor Number... 

Although most water supplies are alkaline due to the presence of dissolved calcium bicarbonate and other minerals, some water supplies have an acidic nature with pH levels within the range of 6.0 to 7.0. Examples are swamp waters containing soluble organic acids (humic or fulvic acids) as a result of degradation of biological matter, or lean waters from granite and other hard rock areas. 

Recognition of fluoride in aqueous media is particularly difficult due to the strongly hydrated nature of the anion. Shinkai and co-workers have demonstrated that ferrocene-boronic acid 27 acts as a selective redox sensor for fluoride which operates in H20 [23]. The favourable interaction between boron and fluoride (a hard acid and hard base, respectively) generates a stability constant of 700 M"1 for the fluoride-ferrocenium complex. Stability constants for both the bromide and chloride complexes are <2 M"1. 

We may conclude at this point that the concept of soft and hard acids and bases (SHAB), as applied normally to equilibria, has to be modified in chemical reactions for the extent of bond formation in the transition state. The soft and hard behaviour is consequently not necessarily reflected in the magnitude and sign of the selectivity coefficient, but rather by the nature of the variation in the selectivity coefficient with the structure (increasing hardness) of the acid (or electrophile). 

Butler also suggested that it is the resulting C02 pressure that defreezes the stuck stopcocks. Additionally, he suggested that there may be some interaction between the carbonic acid and various sites on the glass surface. On the other hand, the acidic nature of fresh soda (any fresh soda) is sufficient to dissolve lime and other hard water deposits. Thus, if the joint is frozen due to such deposits, a fresh soda could ostensibly remove the deposit and defreeze the joint. Despite the loss of a good soda, it would be safer, but probably not as effective, than hydrochloric acid. 

The concept of hard and soft acids and bases is useful in classifying reagents. The nature of the outer electron shell of an atom determines its reactivity. If the electron shell is firmly bound and the orbitals are rigidly directed and of low polarizability, then the atom is said to be hard. If the orbitals are less rigidly held and are more polarizable, the atom is said to be softer. Hard acids tend to react with hard bases, and soft acids react with soft bases. Some typical hard and soft acids and bases are given in Table 1.5. 

This mixed ligand coordination where carboxylate and pyridyl groups are bound to the same cation is not common. In fact, one could argue that this is in contrast to Pearson s hard/soft acid-base distinction in that the uranyl cation is a hard acid and the dipyridyl linkers are softer in nature. Indeed, U02 -pyridyl coordination (2,2 -bipyridine, terpyridines, etc) is quite rare, even in molecular materials, yet in these extended topologies, it is likely that sterics play a considerable role and that the 4,4 -dipyridyl is uniquely suited to displace H2O groups and serve as a bridging moiety. This is mentioned in this context as... 

A general rule is that natural rubbers have better mechanical properties than the synthetic rubbers but the latter have better corrosion resistance. Natural rubbers are superior in certain applications such as with wet chlorine and hydrochloric acid. Natural rubber-based ebonite provides good resistance for such application at higher temperatures up to 90 °C. Corrosion resistance increases with increasing hardness, from a range of 60 on the Shore A scale to 80 on the Shore D scale. Higher proportions of sulfur increase the hardness range in the Shore D scale. 

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