Soft principles

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... 

Coulombe, P. A., and Omary, M. B. (2002). Hard and soft principles defining the structure, function and regulation of keratin intermediate filaments. Curr. Opin. Cell Biol. 14, 110-122. 

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 there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

T. L. Ho, Hard and Soft Acids and Bases Principle in Organic Chemisty, Academic Press, New York, 1977. 

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. 

In table 2 and 3 we present our results for the elastic constants and bulk moduli of the above metals and compare with experiment and first-principles calculations. The elastic constants are calculated by imposing an external strain on the crystal, relaxing any internal parameters (case of hep crystals) to obtain the energy as a function of the strain[8]. These calculations are also an output of onr TB approach, and especially for the hep materials, they would be very costly to be performed from first-principles. For the cubic materials the elastic constants are consistent with the LAPW values and are to within 1.5% of experiment. This is the accepted standard of comparison between first-principles calculations and experiment. An exception is Sr which has a very soft lattice and the accurate determination of elastic constants is problematic. For the hep materials our results are less accurate and specifically in Zr the is seriously underestimated. ... 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

Principles The process consists of tumbling the metal to be coated with a powder of the coating metal. It is considered that a form of welding is involved, but the type of conditions conducive to successful deposition indicates that the deposit adheres by mechanical keying (A/, must therefore be relatively soft) and adhesive forces. Thus pretreatments such as abrading or pickling enhance the keying effect, and the use of a soft metal... 

A general principle may now be stated which permits correlation of the complexing ability of metals Hard acids tend to associate with hard bases and soft acids with soft bases . This statement must not, however, be regarded as exclusive, i.e. under appropriate conditions soft acids may complex with hard bases or hard acids with soft bases. 

The low stability of the complex 3.6 is consistent with the hard and soft acids and bases principle of Pearson (1963, 1968 Parr and Pearson, 1983 theoretical aspects Pearson, 1989 Chatteraj et al., 1991 monograph Ho, 1977). According to that principle hard acids will tend to complex with hard bases and soft acids with soft bases. Water is a hard base, whereas the nitrosyl ion is classified by Pearson as a borderline acid with a tendency to be soft. 

Nucleophilic catalysis is also observed with iodide ions. Fluoride ion does not form nitrosyl fluoride under diazotization conditions, as is to be expected from Pearson s hard and soft acids and bases principle which was discussed briefly in Section 3.2. More recently, nucleophilic catalysis has also been shown to occur with thiocyanate ion (SCN ), thiosulfate ion (HS2Of), dimethyl sulfide, and thiourea (H2NCSNH2) or its alkyl derivatives (see below). 

Hammett equation(s) 78, 93, 148ff., 151 f., 153ff., 167f., 190, 193, 196, 297, 299, 308, 312, 375, 381, 392, see also Dual substituent parameter, and Quantitative structure-reactivity relationships Hammond postulate, in additions of nucleophiles to diazonium ions 157 Hard and soft acids/bases principle (Pearson) 49, 54, 109... 

The principle of hard and soft acids and bases and the problem of competitive coordination in complex compounds. A, D. Garnovskii, D. A. Osipov and S. B. Bulgarevich, Russ. Chem. Rev. (Engl. Transl), 1972, 41, 341-359 (441). 

---