Soft 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). 

A soft Lewis base has a large donor atom of high polarizability and low electronegativity. Iodide ion has its valence electrons in large a = 5 orbitals, making this anion highly polarizable and a very soft base. Other molecules and polyatomic anions with donor atoms from rows 3 to 6 are also soft bases. To summarize, the donor atom becomes softer from top to bottom of a column of the periodic table. 

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. 

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. 

According to the hard and soft acids and bases (HSAB) principle, developed by Pearson in 1963232,233, Lewis acids and Lewis bases are divided into two groups hard and soft. Pearson correlated the hardness of acids and bases with their polarizability, whereby soft acids and bases are large and easily polarizable, and vice versa. A selected list of Lewis acids ordered according to their hardness in aqueous solution is presented in Table 18. The HSAB principle predicts strong association of like partners. Hard acid-soft base complexes mainly result from electrostatic interactions, while soft acid-soft base complexes are dominated by covalent interactions. 

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. ... 

These donor numbers provide an interesting comparison of the relative donor abilities of the various solvents (Table 10.4). ranging from the practically nonpolar 1,2-dichloroethane to the highly polar hexamethylphosphoramide. ((CHdjNIjPO. Note, however, that there is no exact correlation between donor number and permittivity. Some solvents with relatively high permittivities such as mtromethane and propylene carbonate (e/e = 38.6 and 65.1) may be very poor donors (DN — 2.7 and 15.1). Conversely, the best donors do not always have high permittivities pyridine (DN -33.1. e/e0 = 12.3) and diethyl ether (DN = 19.2. e/% = 4.3). This should serve lo remind us that solubility is not merely an electrostatic interaction but that solvation also involves the ability to form covalent donor bonds. Note that pyridine may be considered to be a relatively soft base (Chapter 9). Gulmann has extended the concept... 

Although it would be predicted that nitrogen, as a soft base, would be a less effective ligand than phosphorus (Table 2b), more complexes involving nitrogen as a ligand have been... 

Note that the nPt values in Table 4.5, which give relative rates for Reaction 4.24 with various bases in methanol, are highest for very soft bases [e.g., (C6H5)3P,... 

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. 

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). 

It is seen from Table I that the number of possible specific poisons is very restricted, and, in practice, it may become even more restricted, since some of the listed compounds will not fulfill satisfactorily the criteria for specific poisons. Thus, the applicability of soft bases such as ketones or nitriles will certainly be very scarce. The electron-acceptor molecules that tend to adsorb on electron-... 

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. ... 

Poe and co-workers have extensively studied the thermodynamics, kinetics and the trans effect in the reactivity of such complexes.623-634 635 638,64Cl-650-653 The thermodynamics of the successive anations of [Rh(en)2X(H20)]"+ by halogens suggests that the Rh center is a marginally soft acid, and that its softness is increased by the coordination of a soft base (such as I- ) in a position trans to the reaction site a halide ligand weakens the metal-ligand bond trans to itself, and an I- causes a more dramatic weakening than a Br-, which has more effect than a coordinated q-.634,638-640 The spectrophotometrically obtained thermodynamic parameters are summarized in Table 38. 

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