Soft interaction

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... 

Lewis acid and the ligand. By contrast, soft-soft interactions are mainly enthalpic in origin and are characterised by a negative change in entropy" ... 

One prominent example of rods with a soft interaction is Gay-Berne particles. Recently, elastic properties were calculated [89,90]. Using the classical Car-Parrinello scheme, the interactions between charged rods have been considered [91]. Concerning phase transitions, the sohd-fluid equihbria for hard dumbbells that interact additionally with a quadrupolar force was considered [92], as was the nematic-isotropic transition in a fluid of dipolar hard spherocylinders [93]. The influence of an additional attraction on the phase behavior of hard spherocylinders was considered by Bolhuis et al. [94]. The gelation transition typical for clays was found in a system of infinitely thin disks carrying point quadrupoles [95,96]. In confined hquid-crystalline films tilted molecular layers form near each wall [97]. Chakrabarti has found simulation evidence of critical behavior of the isotropic-nematic phase transition in a porous medium [98]. 

These concepts play an important role in the Hard and Soft Acid and Base (HSAB) principle, which states that hard acids prefer to react with hard bases, and vice versa. By means of Koopmann s theorem (Section 3.4) the hardness is related to the HOMO-LUMO energy difference, i.e. a small gap indicates a soft molecule. From second-order perturbation theory it also follows that a small gap between occupied and unoccupied orbitals will give a large contribution to the polarizability (Section 10.6), i.e. softness is a measure of how easily the electron density can be distorted by external fields, for example those generated by another molecule. In terms of the perturbation equation (15.1), a hard-hard interaction is primarily charge controlled, while a soft-soft interaction is orbital controlled. Both FMO and HSAB theories may be considered as being limiting cases of chemical reactivity described by the Fukui ftinction. 

The quantity b has the dimension of a volume and is known as the excluded volume or the binary cluster integral. The mean force potential is a function of temperature (principally as a result of the soft interactions). For a given solvent or mixture of solvents, there exists a temperature (called the 0-temperature or Te) where the solvent is just poor enough so that the polymer feels an effective repulsion toward the solvent molecules and yet, good enough to balance the expansion of the coil caused by the excluded volume of the polymer chain. Under this condition of perfect balance, all the binary cluster integrals are equal to zero and the chain behaves like an ideal chain. 

This type of fully local potential has some limited use, e.g., to consider adsorption in a slowly varying external potential. It fails, however, to describe the most important phenomena such as surface tension and adsorption at most types of interfaces. These phenomena reflect in a fundamental way the nonlocal interactions in the fluid. The most obvious nonlocality of the free energy arises due to the range of the attractive or soft interactions represented by the second term in the equation of state, —The corresponding potential energy can be obtained by the functional... 

An unusual observation was noted when ethanolic solutions of 2-alkyl-4(5)-aminoimidazoles (25 R = alkyl) were allowed to react with diethyl ethoxymethylenemalonate (62 R = H) [92JCS(P1)2789]. In addition to anticipated products (70), which were obtained in low yield ( 10%), the diimidazole derivatives (33 R = alkyl) were formed in ca.30% yield. The mechanism of formation of the diimidazole products (33) has been interpreted in terms of a reaction between the aminoimidazole (25) and its nitroimidazole precursor (27) during the reduction process. In particular, a soft-soft interaction between the highest occupied molecular orbital (HOMO) of the aminoimidazole (25) and the lowest unoccupied molecular orbital (LUMO) of the nitroimidazole (27) is favorable and probably leads to an intermediate, which on tautomerism, elimination of water, and further reduction, gives the observed products (33). The reactions of amino-imidazoles with hard and soft electrophiles is further discussed in Section VI,C. 

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. 

Although many applications of the hard-soft interaction will be presented in later chapters, two additional applications will be illustrated here. First, consider the interaction of the Lewis acid Cr3+ with the Lewis base SCN, which could donate an electron pair from either the S or N atom ... 

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. 

Disproportionation of BrF occurs with the formation of Br2 and BrF3 or BrF5. Little is known about IF because it so unstable and disproportionates to I2 and IF5. Because of the favorable hard-soft interaction of Ag+ and I, IF can be obtained by the reaction... 

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... 

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