Soft bases and

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

It is important to realize that there is a great deal of overlap in the topics covered in this chapter. For example, the chemistry of metal carbonyls is intimately related to metal alkene complexes, because both types of ligands are soft bases and many complexes contain both carbonyl and alkene ligands. Also, both areas are closely associated with catalysis by complexes discussed in Chapter 22, because some of the best-known catalysts are metal carbonyls and they involve reactions of alkenes. Therefore, the separation of topics applied is certainly not a clear one. Catalysis by metal complexes embodies much of the chemistry of both metal carbonyls and metal alkene complexes. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

One application of the first rule given above is found in complexes between alkenes or aromatic compounds and metal ions (p. 80). Alkenes and aromatic rings are soft bases and should prefer to complex with soft acids. Thus, Ag, Pt2+, and Hg2+ complexes are common, but complexes of Na+, Mg2, or Al3t are rare. Chromium complexes are also common, but in such complexes the chromium is in a low or zero oxidation state (which softens it) or attached to other soft ligands. In another application, we may look at this reaction ... 

Soft acids prefer to associate with soft bases, and hard acids prefer soft bases, and hard acids prefer to associate with hard bases. 

Answer 13.8 Tertiary phosphines are soft bases and actinides are hard acids. This is likely to be the weakest linkage thermodynamically. 

Acidity and basicity are relative properties. Many compounds are amphoteric and behave as acids or as bases according to a partner. Metal oxides are classified as acidic, amphoteric or basic. Experimentally, this classification corresponds to the adsorption of probe molecules[7, 8]. NH3 is a base probe molecule that reacts with the electron deficient metal atoms (Lewis acid) or the protons adsorbed on the hydrated surface, CO2 is usually considered as acidic and thus it is expected to adsorb more strongly on basic sites. According to this classification, Ti02 belongs to an amphoteric species and MgO to a basic species. A general difficulty for such classifications is that the order can vary with the choice of the probe. The Hard and Soft Bases and Acids theory[9, 10] responds to the necessity to refine the model with a second scale it is better to couple... 

Because olefins are soft bases and most Friedel-Crafts halides are hard acids, the primary interaction between these two types of compounds must be regarded as a weak one, the outcome of which is nerally limited to the equilibrium formation of the relatively feeble rr-complex. Only with the softer of the strong Lewis acids would one expect this interaction to proceed further and give direct Hunter-Yohe initiation titanium tetrachloride seems to comply with such a requirement, as suggested by the experimental evidence discussed in Sect. IV-B4-b). 

In the case of the substitution reaction, the attacking species is acting like a nucleophile, which means that it is acting as a soft base, and so it attacks the softer centre, which in this case is the 8+ carbon. However, in the case of the elimination reaction, the attacking species is acting as a hard base, and thus attacks the harder centre, which is the a-hydrogen. The a bond electrons in the carbon/hydrogen bond then act in a similar manner to an internal substitution... 

But if ML is soft and Y is soft, then everything works in favor of a large negative value for AH°, and very stable complexes. Ahrland has made a detailed study of the available data and has found a remarkable agreement with the above predictions. Hard acids rarely form complexes with soft bases, and hard bases do not form very stable complexes with soft acids, except for strong bases such as OH-. 

Soft acids bond preferentially with soft bases and hard acids bond preferentially with hard bases. 

However, a reduction in the complexation ability of the melt-anion leads to a shift in the equilibrium (3.6.3) to the left, which finally results in a reduction in the metal-oxide s solubility. The substitution of bromide ions by iodide ions (transfer from bromide to iodide-melts) leads to the strengthening of the basic properties (which cause a decrease in the oxide solubility) and of the complexation ability (which results in the solubility increase). For molten Csl, the simultaneous action of both factors leads to an increase in the metal-oxide solubilities. However, iodide ion belongs to the group of soft bases, and this is the main reason for the solubilities of oxides in iodide-melts remaining lower than that in the molten alkali-metal chlorides. 

Phosphines PX3 (X = alkyl or aryl, RO-, halogen) generally exhibit a distinct preference for coordination with heavy Group VIII metals, i.e., those defined by Chatt, Ahrland, and Davies (51) as having class b character. Alternatively, using Pearson s classification (259), phosphines are considered to be soft bases and thus readily coordinate with soft acids, e.g.. Group VIII metal ions and other transition metals in low- or zero-oxidation states. 

Table 3.11. Some hard and soft bases and acids occurring in waters...

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