Soft cation

D and 8 0 data on fluid inclusions and minerals at main stage of epithermal Au-Ag mineralization clearly indicate that the dominant source of ore fluids is meteoric water. Meteoric water penetrates downwards and is heated by the country rocks and/or intrusive rocks. The heated water interacts with country rocks and/or intrusive rocks and extracts sulfur, Au, Ag and other soft cations (e.g., Hg, Tl) from these rocks. If hydrothermal solution boils, it becomes neutral or slightly alkaline, leading to the selective leaching of soft cations such as Au, Ag, Hg and Tl from country rocks. However, a contribution of sulfur gas and other components from magma cannot be ruled out. 

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. 

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+. 

Finally, Shibasaki and co-workers reported the use of acetonitrile as a C-nucleophile by cooperative activation with a soft cationic ruthenium catalyst, DBU as base, and a sodium salt (Scheme 76). 

For soft cations, such as Ag+ and Pb2+, covalent contributions are much more important, and consequently the observed order of complex stabilities is quite different from that for alkali cations NH > O > S for Pb2+ and NH, S > O for Ag+. Dissection of the overall effect into enthalpy and entropy contributions (Table 15) reveals the complicated nature of the heteroatom effect. For K+ and Ba2+, the more favourable entropy contribution for N and S ligands is more than offset by the unfavourable change in enthalpy of binding. 

Procedure (d) (cf Section 9.2.4.2), which is useful for alkylations, was not appH-cable to aminations because of coordination of Cu to the amine. Further screening of the salts of soft cations revealed that the addition of Pb salts in conjunction with base activation of the precatalyst led to significantly faster reaction rates. The reaction could be accompHshed with catalyst loading as low as 0.4mol% (compare entries 14 and 15). 

Potentially toxic compounds in the subsurface, such as Cd ", Pb ", or Hg ", which are generally found in very low concentrations, are considered soft cations (Buffle 1988). These ions have strong affinity to intermediate and soft ligands and therefore bond to them covalently. Borderline cations, which embrace transition metals like Cu and Ztfexhibit affinity for the soft cations as well as for alkaline-earth compounds. The order of donor atom affinity for soft metals is O < N < S. Functional groups present in subsurface organic matter that show affinity for soft and borderline metals are shown in Table 14.2. 

Pt(II)and Pt(IV) are both soft cations, showing low affinities for hard ions and atoms (F, O) and high affinities for softer ions and atoms (Cl, Br, I, S, N) and for ligands that can pi bond. Practically all compounds of Pt can be reduced to Pt metal by ignition or moderately strong reducing agents. 

The HSAB (hard and soft acids and base) principle is that hard acids prefer to interact with a hard base, and soft acids with soft bases. Hard bases are not polarizable, and inclnde those with 0-donor atoms. Soft bases are more polarizable, and inclnde S-donor bases. Solvent hardness/softness can be assessed by comparing the Gibbs free energy of transfer of a soft cation like Ag from hard water to the solvent with the Gibbs free energy of transfer of similarly sized hard cations like Na and K. Table 3.9 shows some solvents listed in increasing softness. ... 

Mechanistic studies based mainly on metal ion rescue experiments have identified six oxygen atoms involved in metal ion coordination in the active site (the oxygens in bold font in Figure 19) . Metal ion rescue experiments substitute a potential oxygen ligand with a soft atom, usually sulfur, that is much less inclined to coordinate a hard Mg + ion. If the addition of a soft cation such as Cd + restores activity, the oxygen... 

It is instructive, for example, to compare the structural chemistry of and Zn + which have identical charges and similar sizes. Mg + is hard and rarely occurs in other than six coordination (bonding strength of 0.33 vu), but, because Zn + has a filled d shell which can readily mix with the electrons of the valence shell, it is soft and is found equally often in four- and six-coordination (bonding strength between 0.33 and 0.50, average 0.40 vu, see Section 6.5). A more complete discussion of the behaviour of soft cations is given in Chapter 8. 

In Section 4.5 it was shown that the bonding strengths of soft cations are less well defined than those of hard cations since they are able to form stable compounds with anions having a wider range of anion bonding strengths. Consequently they also display a wider range of coordination numbers. Thus the soft Zn + cation is found in four and six coordination in contrast to the similar, but hard, cation which is usually found only in 6-coordination. Most hard... 

The nature of the donor atom also plays an important role in stabilizing complexation. Substituting one or more sulfur atoms for oxygen in [18]crown-6 (1) causes a decrease in Kt for K+ complexation and an enhanced binding of soft cations such as Ag+. A similar substitution by nitrogen atoms results in a smaller reduction in K+ binding but an increased Ks for transition metal complexation. 

These observations have been interpreted in terms of the hard-soft acid-base theory (77CJC4112), in which the salts of the harder cations, such as the Li+ ion, lead to C-alkylation, whilst the salts of the soft cations, such as the quaternary ammonium salts, are TV-alkylated. This interpretation is particularly relevant in understanding the reactivity of the heteroaryl-magnesium salts. The Mg2+ ion is a harder cation than the Li+ ion and, with the more strongly associated Grignard compounds, C-alkylation predominates. Generally, the pyrrole... 

Thiometalate ligands are coordinated via sulfur to soft cations such as Cu+, Ag+ and Au+. Thus, it has not yet been possible to prepare any compound of the structural type (c) with MOS3 ligands. Following this principle, using oxotrithiometalates, compounds with a cubane-like structure were deliberately prepared. 

The hydrolysis reactions of thioacetals or thioketals are also accelerated by soft cations such as silver(i) or mercury(n) (Fig. 4-44). 

As for carbanions, the reactivity of anionic non-carbon nucleophiles depends on the cation. The nucleophilicity and basicity of a given anionic nucleophile will usually be enhanced if it does not form strong bonds either with the cation or with the solvent. Hard cations, for example Li+ or Ti4+, will significantly reduce the reactivity of hard anions (RO-, R2N , F ), whereas soft cations (Cs+, Cu+, Pd2+) will form strong bonds with soft anions (RS , I , CN , H , R ) and thereby reduce their reactivity. 

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