Hard-soft principle

The theoretical basis for the hard-soft principle It is worthwhile at this point to discuss briefly some of the theoretical concepts behind the hard-soft... 

Aryl fluorides in general are much more reactive than other aryl halides. However, it is noticeable that the reactivity is sometimes controlled by the hard-soft principle of nucleophiles. Scheme 2.5 indicates some of the results. Soft nucleophiles attack the carbon-bromine bond... 

There remain many gaps in this field. Ligands that are regularly applied to one metal may not feature at all in the cage chemistry of another metal. The tendency in the area is for O-donor ligands to be used with early 3d metals, with nitrogen donors becoming more common as the transition series is traversed. While this obeys the hard-soft principle, it is not clear whether... 

To illustrate this principle, let us consider the reactivity of acyl and alkyl chlorides with amines and thiols [18]. Amines are hard nucleophiles and react quickly with acyl chlorides as hard electrophiles. The reaction of thiols as soft nucleophiles with acyl chlorides is surprisingly slow. The reactivity of amines and thiols with soft alkyl chlorides is inversed. All these patterns of reactivity are explainable by the hard-soft principle. 

In general, and as long as we deal with ionic additions, this process is governed by the hard-soft principle and in the case at hand, having potassium as the countercation together with a soft nucleophile, 1,4-addition can be taken for granted. An inspection of the general picture, however, reveals various possibihties to manipulate the outcome of these reactions (see 86). 

We notice here a quite strong solvent dependence, but although 93 formally appears a violation of the hard-soft principle, a zinc chelate such as 95 could easily explain this outcome. 

While with 462 the hard-soft principle is at work, in the case of 463 it is kinetic versus thermodynamic control that leads to 464 and 465 respectively. 

Although the regioselectivity of Friedel-Crafts acylations upon 1-phenylsulfonylpyrrole is ostensibly determined by the "hard-soft" nature of the catalyst <83JOC3214>, this paradigm may not be the controlling principle in determining the regioselectivity of acylations... 

The concept of hard and soft acids and bases can be used to interpret many trends in chemical reactivity. These trends are summarized in the hard-soft acid-base principle (HSAB principle), an empirical summary of results collected from many chemical reactions studied through decades of research. 

C21-0023. State the hard-soft acid-base (HSAB) principle. Define and give examples of hard and soft acids and bases. 

As already noted in section 1.4.3, geochemical features of ore fluids responsible for base-metal and gold-silver types of deposits are distinct. They are summarized in Table 1.22. The differences in metals concentrated to the deposits and geochemical fectures of ore fluids responsible for both types of deposits are interpreted in terms of HSAB (hard, soft, acids and bases) principle by Pearson (1963, 1968) below. 

This submarine vs. subaerial hypothesis for the origin of the two types of deposits (Kuroko deposits, epithermal vein-type deposits) can reasonably explain the difference in metals enriched into the deposits by HSAB (hard-soft acids and bases) principle proposed by Pearson (1963) (Shikazono and Shimizu, 1992). Relatively hard elements (base metal elements such as Cu, Pb, Zn, Mn, Fe) are extracted by chloride-rich fluids of seawater origin, while soft elements (Au, Ag, Hg, Tl, etc.) are not. Hard elements tend to form chloro complexes in the chloride-rich fluid, while soft elements form the complexes in H2S-rich and chloride-poor fluids. Cl in ore fluids is thought to have been derived from seawater trapped in the submarine volcanic and sedimentary rocks. 

The difference in the kinds of metals enriched in Kuroko, base metal vein-type and precious metal vein-type deposits could be explained in terms of the HSAB (hard, soft, acids and bases) principle (Pearson, 1963). According to this principle, relatively hard cations (base metal (Cu, Pb, Zn, Fe, Mn, Ag) ions) tend to combine preferentially with chloride ion in hydrothermal solution, while soft cations (Au, Ag, Tl, Hg ions etc.) combine with H2S and HS . The differences in salinity of ore fluids in base-metal-rich deposits (base metal vein-type deposits and Kuroko deposits) and base-metal-poor deposits (precious metal vein-type deposits) is also in accordance with the HSAB principle. 

The hard-soft acid-base principle is not restricted to the usual types of acid-base reactions. It is a guiding principle that for all types of interactions species of similar electronic character interact best. We have already seen some applications (such as the relative strength of HF and HI) of this principle, which we will continue to call HSAB, but we now consider a number of other types of applications. 

The hard-soft interaction principle allows us to correctly predict the results of many experiments. For example, suppose an aqueous solution containing Cs+, Li+, l , and I- is evaporated. The solid products of the reaction could be CsF and Lil or Csl and LiF ... 

Based on the principles of bonding related to electronegativity, the element with highest electronegativity should bond best to the one with the lowest electronegativity. That means that CsF should be produced. However, based on the hard-soft interaction principle, the ions of similar electronic character should interact best. The small Li+ ion should bond better to F and the large Cs+ should bond better to I-, exactly as is observed. 

Another example of how the hard-soft interaction principle applies to precipitation can be seen in a familiar case from analytical chemistry. Because ions of similar size and magnitude of charges precipitate (interact) best, a good counterion for precipitation of Ba2+ is one that is of similar size and has a —2 charge. In accord with this, Ba2+ is normally precipitated as the sulfate because of the favorable size and charge of the anion compared to the cation. 

The hard-soft acid-base principle just illustrated is one of the most useful principles in all of chemistry for predicting how many types of interactions occur, ft is not restricted to acid-base interactions, so it is better called the hard-soft interaction principle, ft predicts that hard acids (high charge, small size, low... 

Earlier it was described how PH3 is a much weaker base than NH3. That is certainly true when the interaction of these molecules with H+ is considered. However, if the electron pair acceptor is Pt2+, the situation is quite different. In this case, the Pt2+ ion is large and has a low charge, so it is considered to be a soft (polarizable) Lewis acid. Interaction between Pt2+ and PH3 provides a more stable bond that when NH3 bonds to Pt2+. In other words, the soft electron acceptor, Pt2+, bonds better to the softer electron donor, PH3, than it does to NH3. The hard-soft interaction principle does not say that soft Lewis acids will not interact with hard Lewis bases. In fact, they will interact, but this is not the most favored type of interaction. 

Although sometimes referred to as the hard-soft acid-base theory, it is actually a principle that relates to many types of chemical interactions. It provides a good explanation of why HF is a weak acid. If H+ might potentially interact with either H20 or F, the situation with regard to the preferred bonding mode could be shown as follows ... 

The hard-soft interaction principle gives a qualitative explanation for the fact that equilibrium for the reaction... 

The products of a large number of reactions of many types are correctly predicted by the hard-soft interaction principle. Some examples are as follows. 

In the first of these reactions, I is softer than F and As is softer than P. Therefore, the exchange takes place to provide a more suitable match of hard-soft properties. In the second reaction, Mg2+ is a small, hard ion, whereas Ba2+ is much larger and softer. The O2- ion bonds better to Mg2+, whereas S2-bonds better with Ba2+. The hard-soft interaction principle predicts correctly the direction of many reactions of diverse types. 

Ho, Tse-Lok (1977). Hard and Soft Acid and Base Principle in Organic Chemistry. Academic Press, New York. The applications of the hard-soft acid base principle to many organic reactions. 

Pearson, R. G. (1966). /. Chem. Educ. 45, 581. A general presentation of the hard-soft interaction principle by Pearson. 

Polyatomic species containing atoms from group IVA are produced by reducing the elements in liquid ammonia that contains some dissolved sodium. In accord with the hard-soft interaction principle (see Chapter 9), isolation of species containing large anions is best accomplished when a large cation of... 

The formation of sodium chloride is a strong driving force in this reaction. The hard-soft interaction principle (see Chapter 9) is convenient in this case because of the favorable interaction of Na+ with Cl A Other examples of this type of reaction are the following ... 

Known as the thermite reaction, this process is so strongly exothermic that the iron is produced in the molten state. In this case, the replacement Fe3+ by Al3+ is very favorable because Al3+ is a smaller, harder, less polarizable ion, so this reaction is in agreement with the hard-soft interaction principle (see Chapter 9). 

Large cations give a favorable match of cation and anion characteristics, so in accord with the hard-soft interaction principle, the salts that have been isolated contain ions such as R4P+. Because of having an unshared pair of electrons, the SnX3" complexes can function as Lewis bases. 

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

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