HSAB behavior

Lewis acid-base interactions, 42 89-143 change of HSAB behavior, 42 104 chelate ring size and metal ion selectivity, 42 109-118... 

These correlations mean that the HSAB principle could be a useful approach to evaluate the geochemical behavior of metals and ligands in ore fluids responsible for the formation of the epithermal vein-type deposits. Among the ligands in the ore fluids, HS" and H2S are the most likely to form complexes with the metals concentrated in the gold-silver deposits (e.g., Au, Ag, Cu, Hg, Tl, Cd), whereas Cl prefers to form complexes with the metals concentrated in the base-metal deposits (e.g., Pb, Zn, Mn, Fe, Cu, and Sn) (Crerar et al., 1985). 

Figures 11(a) and 11(b) [112] show the variation of Ni-Ge-P deposition rate and Ge content as a function of aspartic acid and Ge(IV) concentration, respectively. A relatively low P content, ca. 1-2 at%, was observed in the case of films exhibiting a high concentration of Ge (> 18 at%). Like other members of its class, which includes molybdate and tungstate, Ge(IY) behaves a soft base according to the hard and soft acids and bases theory (HSAB) originated by Pearson [113, 114], capable of strong adsorption, or displaying inhibitor-like behavior, on soft acid metal surfaces. In weakly acidic solution, uncomplexed Ge(IV) most probably exists as the hydrated oxide, or Ge(OH)4, which, due to acid-base reactions, may be more accurately represented as [Gc(OH)4 nO ] ".

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 ... 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

This behavior can be rationalized if one assumes that in these alkylations the carbon atom of the fluorosulfate has considerably more positive character than does that of methyl iodide and is therefore in HSAB (Pearson, 1969) terms a... 

There would seem to be two positions one can take with respect to the interpretation of the behavior revealed by Figs 1 and 2. The first, which would undoubtedly be favored by proponents of HSAB, is that the large deviations of the points for soft-base nucleophiles in Fig. 2 show that HSAB considerations do play an important role in determining the relative order of reactivity of a series of nucleophiles in nucleophilic substitutions at different electrophilic centers when those centers differ significantly in their degree of hardness , and that the failure to observe sizeable deviations from the correlation line in Fig. 1... 

In order to clarify the different behavior of anion 2 and 3 (Scheme 4.10) toward DMC, various anions with different soft/hard character (aliphatic and aromatic amines, alcohoxydes, phenoxides, thiolates) were compared with regard to nucleophilic substitutions on DMC, using different reaction conditions. Results were in good agreement with the hard-soft acid-base (HSAB) theory. Accordingly, the high selectivity of monomethylation of CH2 acidic compounds and primary aromatic amines with DMC can be explained by two different subsequent reactions, which are due to the double electrophilic character of DMC. The first... 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

Sulfonimidothiazolines, 317, 318 Sulfonyl halides, with 2-aminothiazoles, 69 Sulfur (Sg), in synthesis of, A-4-thiazoline-2-thione, 373 Sulfur compounds, 69 Sulfuric acid, with alcohols, 38, 47, 80, 90 in bromination, 77 dealkylation by, 39 deuterated, 92 diazotization with, 66 in nitrations, 72 rearrangements in, 73, 113 Sulfuryl chloride, 432 Symbiotic behavior, see HSAB theory Symmetry. C2v and C2p, 120 Synthetic fibers, 154... 

In acidic solutions, organic amines protonate to form cations with hydrophobic tails. These ions will seek out and blanket cathodic surfaces, much as carboxylate anions seek out and cover anodic spots in neutral or basic media. The usual choices include amylamine (C5H11NH2), cyclohexyl-amine (C6H11NH2), pyridine (C5H5N), and morpholine [0(CH2CH2)2NH]. Metallic iron behaves as a soft acid in terms of the HSAB classification (Section 2.9), despite the indubitable hard behavior of its trivaJent ion, and so molecules with soft donor atoms adsorb more strongly than hard bases (S > N > 0). This principle can be applied to the design of inhibitors. 

The principle of hard and soft acids and bases [I] (HSAB), and the principle of electronegativity equalization [2], together with frontier orbital theory [3] have been, over the years, very useful to establish the behavior of molecules under different circumstances, their reactive sites, and possible reaction mechanisms [4]. Through these principles, and through the values of the parameters associated with them hardness, softness, and electronegativity, it has been possible to correlate and to analyze experimental information that allows one to characterize the interactions involved between different chemical species in many different situations. From this exp>erience it has been possible to establish a priori the development of a wide variety of chemical reactions. 

The purpose of this work is to start from the basic equations of density functional theory to describe the changes in the energy associated with the transition from one ground-state to another, in terms of different sets of variables. In this process one will find the natural definitions of the hardness and softness kernels, the local hardness, the local softness, the global hardness and the global softness [23]. Then, we will proceed to establish their relation with ionization potentials and electron affinities, in order to confirm their behavior as a measure of chemical hardness or softness [14, 24]. Finally, this theoretical framework will be used to analyze the maximum hardness and the HSAB principles. 

Equations (53) and (55) show opposite behavior with respect to the relative values of riS for a given tia. In Eq. (53), for a given tia the smaller tib the better, while in Eq. (55) for a given r A, the larger rjl the better. Since the sum of Eqs. (53) and (55) leads to the total interaction energy, one can see that the best value of is neither to be much smaller than, nor to be much larger than T a-A natural choice then would be the average of these two situations, r) r)A, which is precisely the HSAB principle. 

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