Oxygen-based soft

Chemical Properties. Although the chemical properties of the trivalent lanthanides are quite similar, some differences occur as a consequence of the lanthanide contraction (see Table 3). The chemical properties of yttrium are very similar too, on account of its external electronic stmcture and ionic radius. Yttrium and the lanthanides are typical hard acids, and bind preferably with hard bases such as oxygen-based ligands. Nevertheless they also bind with soft bases, typicaUy sulfur and nitrogen-based ligands in the absence of hard base ligands. 

It is appropriate at this point to summarize the tendency of various nucleophiles to add to the carbonyl group. In general, the strong bases (organometallics, hydrides, negative ions) are most effective among the neutral nucleophiles, the soft ones, for example the sulfur bases, tend to be more effective in addition than the hard ones, for example the oxygen bases. 

Consistent with the definition of hard and soft metal ions see Hard Soft Acids and Bases) based on the nature of the stable complexes that they form with different ligand donor atoms, these metal ions behave similarly toward donor atoms of nucleic acids and their derivatives (Figure 8). Hard monovalent cations (M+) usually interact with nucleic acid polyanions only in a diffuse ion atmosphere manner, whereas hard and borderline polyvalent (M- +) cations can form both outer- and inner-sphere complexes. Soft metal ions tend to form inner-sphere complexes, however. Hard metal ions (class A) prefer O-donor ligands (usually phosphate oxygens), while soft ones (class B) prefer N-donor atoms of the nucleic acid bases as well as S atoms... 

Fig. 9.8 Comparison of relative ligand dissociation energies for Mn and Co. Zero points for the scales have been arbitrarily chosen. Note that for the. wfi ligands, MeSH, HCN, EtSH. and Me2S, the data points lie above and to the left of those for the oxygen bases. This indicates relatively stronger hard-hard bonding (0—Mn ) or soft-soft bonding (S—Co ), or both. [Modified from Jones. R. W. Staley. R. H. J. Phys. Chem. 1982. 86, 1387. Reproduced with permission.]...

E4.36 Since the tri valent lanthanides and actinides (f-block elements) are found as complexes with hard oxygen bases (i.e., silicates) and not with soft bases such as sulfide, they must be hard. Since they are found exclusively as silicates, they must be considered very hard, unlike the borderline behaviour of Zn(II) (see Exercise 4.33 as well). 

Epoxidation of olefins with peracids (43) is of considerable synthetic value. The intrinsic instability of peracids caused by the soft-hard combination spurs the transfer of an active oxygen to soft bases. 

Peracids readily give up oxygen to soft bases. Thus, perbenzoic acid oxidizes diphenyldiazomethane to yield benzophenone (66), via an initial soft-soft interaction. 

A nice article that describes the importance of oxygen diffusion for silicone-based soft contact lenses may be found online http //www.clspectrum.com/article.aspx7article=12953 Note if the lens contains a hydrophobic surface, it will disrupt the tear flow that results in the deposition of an albumin film on the lens. Not only will this reduce the effectiveness of the lens to correct optical aberrations, but will also cause infection/irritation. Eor more details regarding the lens surface and eye complications, see Rao, J. B., Saini, J. S. Complications of Content Lenses in Contact Lenses. Aquavella, J. V., Rao, G. N., eds. Lippincott Williams and Wilkins Philadelphia, PA, 1987. 

The hard-soft acid-base (HSAB) principle states that hard acids prefer to associate with, and react readily with, hard bases while soft acids prefer to associate with, and react readily with, soft bases. The HSAB principle embodies both kinetic and thermodynamic meaning. Thus, interaction between a Lewis acid and a Lewis base of comparable hardness or softness is predicted to proceed readily and result in the formation of a thermodynamically stable product. Applications of the HSAB principle to coordination chemistry abound.29 For example, DMSO is an ambidentate ligand with both hard (oxygen) and soft (sulfur) donor sites. When complexes are formed with platinum(II), a soft acid, DMSO will typically coordinate via sulfur, while, with the harder acid nickel(II), coordination via oxygen is favored. O... 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

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