Hard anions

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... 

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO,... 

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

It has been shown that in protic solutions, the HOMO of hard anions such as RO" lies at a lower energy (71). Thus increasing the alcohol content of the medium (Table IX) implies a stabilization of the RO" species, and the relative importance of the nucleophile HOMO-substrate supeijacent MO interaction increases (Scheme 10). Because of the structure of the latter (Scheme 11), inversion is favored (Table IX). 

In Scheme 1-71, the reversible reaction is shifted to the right when the anion, X, is larger and the cation, M+, is smaller. For example, this shift to the right is 100% in the presence of Na+, PI v, and only 35% in the presence of Na+, F (Hamon Astruc 1988). The equilibrium takes place as an exchange reaction between the two ion pairs. Reactions of this type are based on the symbiotic-effect premise The interaction between a hard cation and a hard anion or between two soft ions is stronger than that between two ions of different types. 

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. 

Pyridylmethyllithium species generally react as hard anions giving rise to 1,2-addition products when coupled with a,/3-unsaturated carbonyl derivatives. It has been found that the corresponding cyano-Gilman cuprates such as 42 also undergo 1,2-addition to a,/3-unsaturated ketones but do add in a conjugate fashion to cr,/3-unsaturated esters and lactones such as 43 in good yield and stereoselectivity <200314(60) 1843> (Scheme 7). 

Taking the anion as polarizable not only introduces these last terms but also reduces the repulsive energies used compared with those of earlier calculations that took a hard anion according to the mean spherical approximation. By hard anion is meant one that doesn t respond to being squeezed, i.e., is not polarizable. 

The color of salts containing the cations (38) or (40) depends markedly on the anion. Weak donor (hard) anions, for example, AsFg , give yellow (38) or colorless (40) salts, whereas softer anions, for example, Br or I, bestow darker (burgundy or black) colors. 

---