And bond softness

Adams, St. (2001). Relationship between bond valence and bond softness of alkali halides and chalcogenides. Acta Cryst. B57, 278 -87. 

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

The mounting cup (ferrule for botde valves) mechanically joins the valve to the container. The mounting cup may be made from a variety of materials, but is typically tin-plated steel coated on the underside. It contains the gasket which provides the seal. Soft gasketing material is appHed wet and bonded in place or, more frequently in larger cans, cut mbber, polyethylene, or polypropylene gaskets are used. 

Under severe conditions and at high temperatures, noble metal films may fail by oxidation of the substrate base metal through pores in the film. Improved life may be achieved by first imposing a harder noble metal film, eg, rhodium or platinum—iridium, on the substrate metal. For maximum adhesion, the metal of the intermediate film should ahoy both with the substrate metal and the soft noble-metal lubricating film. This sometimes requires more than one intermediate layer. For example, silver does not ahoy to steel and tends to lack adhesion. A flash of hard nickel bonds weh to the steel but the nickel tends to oxidize and should be coated with rhodium before applying shver of 1—5 p.m thickness. This triplex film then provides better adhesion and gready increased corrosion protection. 

The i5p-titanium(IV) atom is hard, ie, not very polarizable, and can be expected to form its most stable complexes with hard ligands, eg, fluoride, chloride, oxygen, and nitrogen. Soft or relatively polarizable ligands containing second- and third-row elements or multiple bonds should give less stable complexes. The stabihty depends on the coordination number of titanium, on whether the ligand is mono- or polydentate, and on the mechanism of the reaction used to measure stabihty. 

FIGURE 8.46 Hyaluronic acid has the consistency of a soft gel. It absorbs water easily and bonds with the proteins in the skin, making it useful in the repair of wounds and joint injuries. 

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

This equation indicates that polarizability is proportional to the inverse square of the excitation energy. Therefore, atoms, molecules, and solids with small values of A are easily polarized. That is, they are chemically and mechanically soft. The gaps in their bonding energy spectra are small. Since they absorb light easily, they tend to be colored. If A lies in a narrow band as in a dye, the coloration is bright and saturated. If it lies in broad band as in adhesive polymers, it may be a muddy brown. 

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. 

Although transition metal-catalyzed allylic alkylation has become one of the most powerful methods in chemical synthesis, the formation of ether bonds using this process has been slow to evolve.119-121 The main reasons for this disparity are the lower nucleophilicity and higher basicity of oxygen nucleophiles, particularly those derived from aliphatic alcohols, compared to their carbon or nitrogen analogs. However, this notion has rapidly been revised, as recent advances in the O-allylation area have largely addressed the issue of the reactivity mismatch between the hard alkoxide and the soft 7r-allylmetal species to provide a considerable body of literature. 

The first symposium on soft and hard (Lewis) acids and bases was organized by Prof. R. F. Hudson (now at the University of Kent, Canterbury) at our institute in May 1965, and the proceedings partly published in Structure and Bonding 1, pp. 207—248. A second symposium was organized by Professor Malcolm J. Frazer, Northern Polytechnic, Holloway, London, 29.—31. March 1967. The proceedings of this discussion were not published, but the present paper is based on the invited lecture, partly modified after further helpful comments by Professors /. Bjerrum, K. Fajans and G. Klopman. 

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