Soft sulfur donors

The formation of rings that contain a thioether linkage does not appear to be catalyzed efficiently by Ru, even when terminal olefins are present. On the other hand, molybdenum appears to work relatively well, as shown in Eqs. 30 [207] and 31 [208]. Under some conditions polymerization (ADMET) to give poly-thioethers is a possible alternative [26]. Aryloxide tungsten catalysts have also been employed successfully to prepare thioether derivatives [107,166,169]. Apparently the mismatch between a hard earlier metal center and a soft sulfur donor is what allows thioethers to be tolerated by molybdenum and tungsten. Similar arguments could be used to explain why cyclometalated aryloxycarbene complexes of tungsten have been successfully employed to prepare a variety of cyclic olefins such as the phosphine shown in Eq. 32 [107,193]. 

Transport of Silver Perchlorate by Lipophilic Monothia- and Dithiacrown Ethers. For lipophilic monothia-15-crown-5 16, dithia-15-crown-5 12 and dithia-18-crown-6 13, transport of Na, Cd ", Hg, Pb and Zn as the perchlorates was undetectable. However, all three macrocycles exhibited efficient transport of silver perchlorate. The Ag transport rates for 16,12 and 13 were 7.92 0.05, 7.52 0.15 and 6.87 0.05 X lO" mole/24 hours, respectivley. Thus the soft Ag cation was transported with almost equal efficiency by mono- and dithiacrown ethers which suggests that only one soft sulfur donor atom interacts with the soft Ag. ... 

Replacement of one or two oxygen donor atoms in a lipophilic 15-crown-5 or 18-crown-6 compound with nitrogen or sulfur atoms strongly influences the type of metal ion species which can be transported through the bulk chloroform membranes. Incorporation of one or two soft sulfur donor atoms provides carriers 12,13 and 16 which transport soft Ag, but not the hard alkali metal cations. 

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... 

The soft S2 donor sets presented by these bidentate ligands lead to very strong binding of heavy metals (Table 7) which are not stripped by sulfuric acid, ensuring that these deleterious elements do not transfer to the Zn electrolyte.196 However, co-extraction of copper is accompanied by reduction to Cu1 which has proved very difficult to strip to regenerate the reagent and will lead to poisoning of the extractant unless all traces of copper are removed from the feed solution. 

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

The thiocyanate ion provides an interesting test of these ideas. In the SCN ion, the sulfur atom is considered to be a soft electron donor, whereas the nitrogen atom is a much harder electron donor. Accordingly, Pt2+ bonds to SCN- at the sulfur atom, whereas Cr3+ bonds to the nitrogen atom. Uncharged metal atoms are considered to be soft electron acceptors, and they form complexes with soft ligands such as CO, 11, and PR3. We will see many examples of such complexes in later chapters. On the other hand, we would not expect complexes between uncharged metal atoms and NH3 to be stable. 

Crown ethers with mixed donor sets of soft sulfur and hard oxygen heteroatoms selectively ligate heavy metal ions such as mercury and silver. Porphyrazines with thia-oxo crown units appended to the periphery were published simultaneously by Hoffman and co-workers (25) and by Nolte and co-workers (26, 27). 

In general, the nucleophilicity pattern is in accord with the soft or Class b requirements of platinum(II). Ligands with light element donors, N, O and F, are considerably less effective than their second-row equivalents. Thus, F" < Cl- < Br < I- R20 < R2S < R2Se and R3N R3P > R3As. Carbon donors, e.g. CNR and CN , are good nucleophiles towards Ptn. The extent to which the nucleophilicity is modified by the substituents attached to the donor has not yet been evaluated thoroughly. Probably the most extensive set of data refers to sulfur donors, the values... 

To further exemplify this methodology, let us take a typical example of the application of a template reaction as seen in the synthesis of a mixed N2S2 donor macrocyclic ligand 6.11. This compound is of interest to the co-ordination chemist as it possesses a potentially square-planar array of soft (sulfur) and harder (nitrogen) donor atoms. What sort of co-ordination chemistry is it likely to exhibit Will the hard or the soft characteristics dominate The most obvious route for the synthesis of 6.11 would involve the reaction of the dithiol 6.10 with l,2-bis(bromomethyl)benzene (Fig. 6-7). 

Barriers of 12.5-15.5 kcal/mol for neutral CpMo(CO)(MeC=CMe)-(SR) complexes are quite similar to rotational barriers in cationic complexes (74). Given the 7r acidity of CO and the tt basicity of SR-, these barriers are surprisingly small. Sulfur donor ligands tend to be electronically flexible, and the soft thiolate may facilitate alkyne rotation by simultaneous rotation of the thiolate substituent. 

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