Lewis acid-base interactions sulfur donors

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+. 

From elemental sulfur to selenium and tellurium, intermolecular interactions (,secondary bonds, soft-soft interactions) play an increasing role. According to N. W. Alcock,1 the term secondary bond describes interatomic distances longer than covalent single bonds but shorter than van der Waals interatomic distances.1 In many cases secondary bonds can also be described as coordinative Lewis base - Lewis acid or charge transfer (donor-acceptor) types of interactions. 

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... 

Despite the fact that sulfonium salts are much more easily prepared than oxonium ones, simple sulfur bases appear to be considerably less basic towards protonic acids, hydrogen bond donors, and most Lewis acids than are their oxygen analogs. The exact values for the freezing point depressions of sulfides in sulfuric acid are controversial (134,136,334) but it is clear that if proper allowance is made for sulfonation, a twofold depression is approached magnifying protonsr tion of the sulfur atom. Evidence for the existence of stable sulfonium salts from the interaction of mercaptans, sulfides and disulfides with hydrogen chloride and antimony pentachloride at low temperatures has recently been described (205). 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

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