Hard bases table

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

The first task is to decide whether the members of a given group are Lewis acids or bases. Then evaluate the relative softness and hardness based on polarizability, taking into account correlations with electronegativity, size, and charge. Refer to the periodic table in assessing the trends. 

These d indices for cations and anions relevant to AB cements are shown in Table 2.5. Bases which add on through F or O and do not form i-bonds have similar hardness values they are hard bases. Soft bases form dative 7i-bonds with many cations. They have high-energy-level occupied orbitals with unshared electron pairs. 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

Hard bases (HB) have low polarizability and strong Br0nsted basicity in the V, VI and VII groups of the periodic table, the first atom is always the hardest and the hardness decreases with increasing atomic number in the group. Typical examples are H20, OH-, F-, O2-, CH3COO, S042-, Cl-,... 

As was pointed out in the previous chapter, biologically important metal ions and their ligands can be classified according to the hard-soft theory of acids and bases (Table 2.1). While there are exceptions, most metal ions bind to donor ligands as a function of preferences based on this concept, with hard acids (metal ions) binding preferentially to hard bases (ligands) and soft acids to soft bases. 

One simple hardness test is the Moh hardness test it is based on the fact that a harder material will scratch a softer material. Geologists and mineralogists frequently use this test. The Moh scale is an arbitrary scale of hardness based on the ability of ten selected minerals to scratch each other. The relative Moh hardness for several substances is given in Table 15.6. 

Using these properties, a number of species have been placed in the hard, soft, or borderhne categories in Table 3.2. This table can be used to predict, at least qualitatively, the strength of complexation as measured by the stability constants. For example, Pu is a hard acid, F, a hard base, and T, a soft base. This leads to the prediction that log PdPuF O would be larger than log PdPuFOi the experimental log (3i values are 6.8 and <1.0, respectively. By contrast, since Cd is a soft acid, log Pi(CdF ) could be expected to be less than log (3i(CdT) the respective values are 0.46 and 1.89. However, many metals of interest such as... 

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. ... 

As well as increasing anion nucleophilicity, crown or cryptand complexation can enhance the basicity of the anion. Table 3 exemplifies this effect with 1-bromooctane where base-promoted elimination to 1-octene competes with nucleophilic substitution. Being small and poorly solvated, naked fluoride is a strong and hard base which causes, in the case of certain substrates (e.g. Scheme 6), the elimination product to predominate. As the naked anions increase in size they display less basic characteristics but retain high nucleophilic reactivity (74JA2250). 

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. 

Ligands with N, O, F, Cl-donor centers, or compounds containing a combination of these elements, are hard bases according to Pearson. On the contrary, all ligand systems, containing elements further to the right of these rows, and their combinations, are soft bases. Specific examples of such bases-ligands are represented in Table 2 and in Chap. 2. 

At the same time, it is necessary to take into account that the approach described has a number of exceptions, related for example to the nature of other ligands forming pseudohalide complexes. A series of classic examples of inversion of the bond M — N —> M — S —> M — N have been reported [6,8,11,42-44,59] and are presented in Sec. 2.2.3.5. In this respect, we especially emphasize the capacity of other ligands for soft or hard metals, related with symbiotic [60] and anti-symbiotic [61] effects. Thus, Pearson [61] emphasized that soft ligands, which are placed in a trans position to SCN ion, contribute to N-binding of thiocyanate ions, and hard bases contribute to S-coordination of these ambidentate ligands. Metal oxidation number (Table 1.4) is important in this problem and it regulates soft hard properties of complex-formers. 

The adsorption of organic compounds on nascent surfaces can be considered as an acid-base reaction. According to the hard-soft acid and bases HSAB principle (Ho, 1977), polar compounds such as carboxylic acid and amine (with lone pair electrons on oxygen or nitrogen) are classified as "hard bases". A hard base reacts more easily with a hard acid than with a soft acid. Metals are classified as soft acids which react much more easily with soft bases than hard bases. The results in Table 5.1 can be explained with this concept. The soft bases (benzene, 1-hexene, diethyl disulfide) react easily with the nascent surface as a soft acid. On the other hand, the hard bases such as propionic acid, stearic acid, propyl amine and trimethyl phosphate exhibit a very low activity (Fischer et al., 1997a and 1997b Mori and Imazumi, 1988). 

In lanthanide complexes, the Ln ions are hard Lewis acids, which prefer to coordinate hard bases, such as F, O, N ligands. The f-orbitals are not involved to a significant extent in M-L bonds, so their interaction with ligands is almost electrostatic in nature. Table 18.3.1 lists some examples of the various coordination... 

Ammonia is a strong Lewis base and it is small in size. The various forms of ammonia expected to occur on surfaces due to specific interactions (see Table I) can be detected by infrared spectroscopy. Interactions should be quite strong since ammonia is a fairly hard base. Thus, criteria a-d (see Section II.C.l) could hopefully be fulfilled. 

As a general rule, the basic atoms associated with soft bases have lower electronegativities and are more polarizable. Likewise, the basic atoms associated with hard bases have higher electronegativities and are less polarizable. Therefore, using the periodic table of the elements, the group of bases listed above can be separated as illustrated. ... 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

Table 4 Solubility principles for salts of hard-base anions...

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