HSAB principle

The mesomeric anion (157 Scheme 10) reacts readily with electrophilic reagents such as alkyl or acyl halides at N-2, C-4 and the exocyclic oxygen atom. The percentages of the different products formed are controlled by the HSAB principle. The acylium ion (hard) attacks preferentially at oxygen (hard), whilst the softer alkylating agents attack the nitrogen atom. 

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. 

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

The HSAB principle predicts that the equilibrium should lie to the right, because the hard acid CH3CO should have a greater affinity for the hard base RO than for the soft base RS . Indeed, thiol esters are easily cleaved by OR or hydrolyzed by... 

Once acids and bases have been classified as hard or soft, a simple rule of the HSAB principle can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases. 

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. 

C21-0007. Iron is always found in nature in compounds, often with oxygen. The other members of Column 8 in the periodic table, mthenium and osmium, occur in elemental form. Explain these observations using the HSAB principle. 

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

These differences can be explained by the HSAB principle by Pearson (1963, 1968). This principle indicates that HS and H2S are likely to form complexes with the metals enriched in the Non-Green tuff-type (Au, Hg), whereas Cl prefers to form complexes with the metals concentrated in the Green tuff-type (Ag, Pb, Mn, Fe, Cu). 

They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

Classification of metals and ligands according to the HSAB principle... 

Shikazono, N. and Shimizu, M. (1992) Associated metals in vein-type deposits in Japan Interpretation using the HSAB principle. Can. Min., 30, 137-143. 

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 use of more polar solvents (acetonitrile, DMF) improved the N 0 ratio in the Mitsunobu alkylation of 3-benzoyl thymine with cyclopentanol <06SL324>. N vs. O-selectivity in the alkylation of 2-pyrimidinones has been investigated and the results rationalised on the HSAB principle <06T6848>. 

One of the simplest applications of the HSAB principle is related to solubility. The rule "like dissolves like" is a manifestation of the fact that solute particles interact best with solvent molecules which have similar characteristics. Small, highly charged particles or polar molecules are solvated best by solvents containing small, highly polar molecules. Large solute particles having low polarity are solvated best by solvent molecules having similar characteristics. Consequendy, NaCl is soluble in water, whereas sulfur, S8, is not. On the other hand, NaCl is insoluble in CS2, but S8 dissolves in CS2. 

The application of the HSAB principle is of considerable importance in preparative coordination chemistry in that some complexes are stable only when they are precipitated using a counterion conforming to the above rule. For example, CuCls3 is not stable in aqueous solution but can be isolated as [Cr(NH3)6][CuCl5]. Attempts to isolate solid compounds containing the complex ion Ni(CN)s3 as K3[Ni(CN)5] lead to KCN and K2[Ni(CN)4]. It was found, however, that when counterions such as Cr(NH3)63+ or Cr(en)33+ were used, solids containing the Ni(CN)53 anion were obtained. 

We have already used the HSAB principle as it applies to linkage isomers in metal complexes. This application to bonding site preference can also be used to show the behavior of other systems. For example, the reactions of organic compounds also obey the principles when reacting with nucleophiles such as SCN- or N02 ... 

Keep in mind that this classification, which so clearly now is a manifestation of the HSAB principle, predated that principle by approximately a decade. In retrospect, it seems almost obvious now. 

One apparent contradiction to the HSAB principle involves the stability of the complexes [Co(NH3)5NCS]2+ and [Co(CN)5SCN]3-. In the first of these, the thiocyanate ion is bonded to Co3+ through the nitrogen atom, as expected. However, in the second complex, SCN- is bonded to Co3+ through the sulfur atom, and this arrangement is the stable one. The difference between these... 

Hard and soft acid and base (HSAB) principle, 16 780 Hard blacks, 21 775 Hard-burned quicklime, 15 28 Hard coals, 6 703 classification, 6 712 Hard copper alloys, 7 723t relief annealed, 7 723t Hard copy systems, 9 513-514 Hard core repulsion, 23 93 Hard-elastic olefin fibers, 11 242 Hardenability, of steel, 23 283—284 Hardened MF resins, analysis of,... 

The next consideration is the HSAB principle formulated at a local level. Let us consider the interaction energy between two chemical species A and B, in which one is electrophilic and the other nucleophilic. From a global point of view and neglecting the effect of change in external potential of A and B, the change in grand canonical potential can be expressed as [7a]... 

If the interaction between A and B occurs through the ith atom of A and the Mh atom of B, then the most favorable situation that arises from the minimization of AOa and AOb leads to, vA, =, vB/,. Hence, the interaction between A and B is favored when it takes place between those atoms whose softnesses are approximately equal. This is essentially the local HSAB principle. 

Application of the local HSAB principle to determine the preferred site of attack... 

The pathway characterized with the lower energy barrier is expected to be the preferred reaction channel, especially when the addition leads to the same product. Following the local HSAB principle, one has to look at the softness matching criteria, and the minimum of sAi —. BJ and sAj —. B will determine the preferred site of attack. 

It is clear from the A.v values in Table 12.2 that the site associated with the lower A.v value, implying a better satisfaction of the local HSAB principle, is the preferred site of attack for this type of reaction [26]. 

These two paths are normally associated with different barrier heights introducing, thus, a regio-selectivity in the cycloadditive process. The path associated with the lower energy barrier should be preferred, and the corresponding cycloadduct will be dominant. Now, direct application of HSAB at the local level is not possible here, because it has to be satisfied for both the termini simultaneously. A softness matching criteria, thus, needs to be defined for the multisite interaction that measures the extent of the fulfillment of local HSAB principle. A quantity (A.v) can, thus, be defined to measure the softness matching criteria for the two paths in a least square sense, and the minimum value of this quantity should be preferable [27] ... 

Applications of local HSAB principle have been used for the determination of the softer regions in Si clusters by using Ga as probe atom [30a], or the site for H-atom adsorption on Si clusters. In the latter case, the isomer predicted by the Fukui function was found but it is not always the most stable one. The use of the reactivity indices is only valid when the adsorption process does not induce strong deformation of the cluster [30b]. 

Local HSAB principle can also be used to calculate the relative homolytic bond dissociation energies (BDE). For the homolytic dissociation of para-substituted phenols ... 

Interaction Energy Using Local HSAB Principle.366... 

---