Hard-Soft Interaction Principle

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. 

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. 

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+. 

Precipitation Making Use of the Hard-Soft Interaction Principle... 

In many parts of this book, the utility of the hard-soft interaction principle has been described. In the... 

As might be expected on the basis of the hard-soft interaction principle (see Chapter 9), large, soft cations form insoluble compounds with these anions. Accordingly, the anions form precipitates with Hg2+, Pb2+, and Ba2+. 

When painting a wall, better coverage is assured when the roller passes over the same area several times from different directions. It is the opinion of the author that this technique works well in teaching chemistry. Therefore, a second objective has been to stress fundamental principles in the discussion of several topics. For example, the hard-soft interaction principle is employed in discussion of acid-base chemistry, stability of complexes, solubility, and predicting reaction products. Third, the presentation of topics is made with an effort to be clear and concise so that the book is portable and user friendly. 

On the basis of the hard-soft interaction principle, predict which species would be more stable, and explain your answer. 

Using the hard-soft interaction principle, explain why the apparent ionic radius of H in LiH is smaller than it is in KH. 

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