Bare Cations

Ziegler, T., Li, J., 1994, Bond Energies for Cationic Bare Metal Hydrides of the First Transition Series A Challenge to Density Functional Theory , Can. J. Chem., 72, 783. 

Superbase systems are known to contain a strong base and a solvent or reactant capable of specifically binding the cation baring the conjugated anion [138]. Such systems can be prepared on the basis of linear or cyclic glycol ethers, microcyclic polyethers (crown ethers), highly polar non hydroxylic solvents (sulfoxides, e.g., DMSO), sulfones (sulfolane), amides (N-methylpyrrolidone, dimethylformamide, hexametapol), and phosphine oxides as well as from liquid anunonia, amines, etc. For example, basicity of sodium methylate in 95% DMSO is by seven orders higher than in pure methanol [139]. 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

The most commonly observed effect of current flow is the development of alkaline conditions at the cathode. On bare metal this alkaline zone may exist only at the metal surface and may often reach pH values of 10 to 12. When the soil solution contains appreciable calcium or magnesium these cations usually form a layer of carbonate or hydroxide at the cathodic area. On coated lines the cations usually move to holidays or breaks in the coating. On failing asphalt or asphalt mastic type coatings, masses of precipitated calcium and magnesium often form nodules or tubercles several centimetres in diameter. 

First, the hydrogen bond is a bond by hydrogen between two atoms the coordination number of hydrogen does not exceed two.7 The positive hydrogen ion is a bare proton, with no electron shell about it. This vanishingly small cation would attract one anion (which we idealize here as a rigid sphere of finite radius—see Chap. 13) to the equilibrium intemuclear distance equal to the anion radius, and could then similarly attract a second anion, as shown in Figure 12-1, to form... 

A hydrogen cation is a hydrogen atom that has lost its single electron, leaving a bare hydrogen nucleus. A bare hydrogen nucleus is a proton. Thus, any reaction in which H moves from one species to another is called a proton-transfer reaction. Protons readily form chemical bonds. In aqueous solution, they associate with water molecules to form hydronium ions. 

This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated... 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. 

Schroder, D., Schwarz, H., 1995, C-H and C-C Bond Activation by Bare Transition-Metal Oxide Cations in the Gas Phase , Angew. Chem. Int. Ed. Engl., 34, 1973. 

DF calculations were carried out on CO complexes of small neutral, cationic, and anionic gold clusters Au with n= 1-6. The -coordination mode (terminal C-coordination) was found to be the most favorable one irrespective of the charge of the cluster, and cluster planarity is more stable for the bare clusters and their carbonyls. As expected, adsorption energies are greatest for the cationic clusters, and decrease with size. Instead, the adsorption energies of... 

As the discussion of Chapter 2 and the numerical charges in (3.190) suggest, the extreme ionic picture such as (3.189a) must be modified by donor-acceptor interactions that create partial covalency by delocalizing significant charge ( 0.5e) from bare fluoride ions into acceptor orbitals of the central cation. Such partial-covalency effects can be represented by resonance delocalization of the form... 

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