Bases hardness, table

The EA/CA ratio was proposed as a measure of hardness of the Lewis acid, and EB/CB as hardness of the Lewis base in aqueous solution (17). It now seems that the E/C ratio is not a measure of hardness in the sense in which Pearson (5,5a) defined hardness. Rather, the E/C ratio for a Lewis acid or base is a measure of the tendency to ionicity in the M-L bonds formed. The EAICA ratio should rather be called IA, and the EbICb ratio IB, the tendency to ionic bonding in forming the M-L bond. Acids and bases in Tables I and II are placed in order of increasing tendency towards ionicity in the M-L bond, according to the E/C ratios IA and 7b. A justification for this interpretation is that the order of IA values for metal ions in aqueous solution strongly resembles the order of hardness derived by Pearson (19) from enthalpies of complex forma-... 

The HSAB (hard and soft acids and base) principle is that hard acids prefer to interact with a hard base, and soft acids with soft bases. Hard bases are not polarizable, and inclnde those with 0-donor atoms. Soft bases are more polarizable, and inclnde S-donor bases. Solvent hardness/softness can be assessed by comparing the Gibbs free energy of transfer of a soft cation like Ag from hard water to the solvent with the Gibbs free energy of transfer of similarly sized hard cations like Na and K. Table 3.9 shows some solvents listed in increasing softness. ... 

One of the most useful tools for predicting the outcome of chemical reactions is the principle of hard and soft acids and bases (HSAB), formulated by Pearson in 1963 [13-15]. This prindple states that hard acids will react preferentially with hard bases, and soft acids with soft bases, hard and soft referring to sparsely or highly polarizable reactants. A selection of hard and soft Lewis acids and bases is given in Table 1.1. 

If is much greater than one, the base is soft. Conversely, if AT is s 1, the base is hard. Table 2.6 shows common hard and soft bases. l The nature of the outer groups on the acceptor atom is important. The hard acid BF3 possesses hard fluoride ions and readily adds to hard bases. This contrasts with BH3, which is a soft acid where soft hydride ions readily add to soft anions. Soft bases tend to group together on a given central atom as do hard ligands. There is a mutual stabilizing effect called symbiosis. ... 

BD. They reported an improvement in shape memory properties as a result of introduction aromatic structure into the main chain. Yang and co-workers [122] compared the mechanical, dynamic mechanical and shape memory properties of PU block coPolymers with planar shape hard segment (1,6-diphenyl diisocyanate (PDI)) and bent shape hard segment (MDI). The PDl-based PU showed superior properties compared with MDI-based PU (Table 2.11) as a result of better interaction among hard segments due to the planar shape of PDI. 

The behavior of Co-based wear resistant alloys is based on a coarse dispersion of hard carbide phases embedded in a tough Co-rich metallic matrix. The volume fraction of the hard carbide phase is comparatively high e.g., at 2.4wt%C the carbide content is 30wt%. The carbide phases are M7C3 (Cr7C3 type) and MgC (WgC type). Table 3.1-85 lists characteristic properties of Co-based hard facing alloys the compositions of which are listed Table 3.1-83. 

Table 3.1-85 Properties of selected Co-based hard-facing alloys...

Based on the hardness results, vulcanizate with gadolinium oxide were characterized by the lowest hardness (Table 15.3). 

Ceramics, particularly new ceramic composites, are widely used in the cutting-tool industry. For example, alumina reinforced with silicon carbide whiskers (extremely fine fibers) is used to cut and machine cast iron and harder nickel-based alloys. Ceramic materials are also used in grinding wheels and as abrasives because of tiieir exceptional hardness (Table 12.4). Silicon carbide is the most widely used abrasive. 

Based on the research of Klopman, and Parr and Pearson (Klopman 1968 Parr and Pearson 1983), Pearson described the absolute hardness (ri) quantitatively as being proportional to the difference between I (ionization potential) and A (electron affinity) of the species (Pearson 1988). Absolute softness is defined as t). The absolute electronegativity (x) and the absolute hardness (Tj) are applied quantitatively to any given acid-base reaction. Table 3.10 presents x and Tj values for some representative metal ions. 

Eq. 5.30 is a general relationship for the interactions of electrophiles and nucleophiles, and is not restricted to definitions and discussions of hard and soft acids and bases. It tells us that the relative nucleophilicity of several Lewis bases will depend upon which electrophile is used, because the c s and yS values will change for each different electrophile. Similarly, the relative electrophilicities of several Lewis acids will depend upon what nucleophile is used. We will see exactly such results when we explore quantitative scales for various nucleophiles and electrophiles, where the scales are highly dependent upon the particular reaction that is chosen to analyze relative reactivities (see Chapter 8). Eq. 5.30 nicely explains the reactivity trends for soft acids and bases. It predicts that the Eoveriap will be best for Lewis acids and bases that have electrophilic and nuclephilic orbitals of roughly the same energy, which is the cases for the soft acids and bases of Table 5.8. 

The group 2 metal ions are hard acids and are preferentially coordinated by hard bases (see Table 7.9). In this section we consider complexes formed in aqueous solution in which the metal centre is coordinated by O- and A -donor ligands to give cationic species. Two important ligands are [EDTA]" (see eq. 7.75) and [PsOjo] (see Fig. 15.19). Both form water-soluble complexes with Mg and the heavier metal ions, and are sequestering agents used in water-softening to remove Mg + and Ca ions. 

FIGURE 3.4 Graphical correlations between the values of the molecular chemical hardness tj and the maximum hardness index Y by employing the Tables 3.4-3.7 for the Lewis acids and bases of Tables 3.1 and 3.2, in the case of experimental finite-difference (FD) based definition of atomic chemical hardnesses of Table 3.3, in left and right pictures, respectively (Putz, 2008c). 

In the same manner, the investigation of Figure 3.6 provides the chemical hardness orderings of Lewis bases of Table 3.2 along the computational scheme implemented (Putz, 2008c) ... 

DCPD built into UPR enhanced the degree of drying of the coating surface and their Persoz pendulum hardness (Table 16). Comparable properties could be achieved for UPRs based on propylene glycol, but the use of dicyclopenta-diene is more effective from the economical point of view. 

The theory was proposed by R. G. Pearson in 1963. " Acids and bases are qualitatively divided into soft and hard , the remaining cases being border cases (see Table 2.7.4) Soft acids and bases mainly form covalent bonds in mutual reactions, while hard acids and bases are mainly bonded by ionic forces. The principal rule is that hard acids react most strongly with hard bases, while soft acids react most strongly with soft bases. Hard metal ions react... 

Table 1 demonstrates the elFeet of OHm x on gel time (tgei) for PUs based on individual polyols with/= 2, 3 and 5.5. Both 1,4-dihydroxybutane (1,4-DHB) and polypropylene glycol (PPG) are linem diols (f = 2), however, of different backbone length and OHi dex (Table 1). Consequently, when cmed imder the same conditions (e.g., at 115 °C), their respeetive gel times were foimd to be drastically different 10 s vs. 680 s. The role of the backbone length was also evident in the distinct physical properties of these materials, estimated via Shore D hardness H) 87 vs. 53. A similm effect of the OHi dex on the cming kinetics and physical property was also observed for pairs of f= 3 polypropylene-oxide based polyether and /= 5.5 sucrose-based polyols (Table 1).

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