Metal Complexes and Anions

FIGURE 14.1 Percentages of TLC/HPTLC publications on metal cations, metal complexes, and anions that appeared during the period 1980 to 2004. 

Acid-base equilibria in aqueous solutions are a source of enormous possibilities in terms of altered (increased or decreased) reactivity for practical purposes or as a tool in the mechanistic toolbox. The same is true for coordination and ion-pairing equilibria involving typically cationic metal complexes and anions, although oppositely charged reactants can also be involved. 

Mollah, S. Pris, A. D. Johnson, S. K. GwizdalaIII, A. B. Houk, R. S. Identification of metal cations, metal complexes, and anions by electrospray mass spectrometry in the negative ion mode. Anal Chem. 2000, 72, 985-991. 

Cyanoacrylate adhesives cure by anionic polymerization. This reaction is catalyzed by weak bases (such as water), so the adhesives are generally stabilized by the inclusion of a weak acid in the formulation. While adhesion of cyanoacrylates to bare metals and many polymers is excellent, bonding to polyolefins requires a surface modifying primer. Solutions of chlorinated polyolefin oligomers, fran-sition metal complexes, and organic bases such as tertiary amines can greatly enhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a critical component of these primers, as solvent swelling of the surface facilitates inter-... 

Two commonly used synthetic methodologies for the synthesis of transition metal complexes with substituted cyclopentadienyl ligands are important. One is based on the functionalization at the ring periphery of Cp or Cp metal complexes and the other consists of the classical reaction of a suitable substituted cyclopentadienyl anion equivalent and a transition metal halide or carbonyl complex. However, a third strategy of creating a specifically substituted cyclopentadienyl ligand from smaller carbon units such as alkylidynes and alkynes within the coordination sphere is emerging and will probably find wider application [22]. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The borole ring and various 1,3-diborolyl anions have been extensively employed as ligands to prepare a huge array of transition metal complexes and multidecker sandwich compounds.96 97 Inevitably, the electronic character of the borole is profoundly changed upon complexation, so a study of such complexes can reveal nothing certain about B—C multiple bonding in the isolated ligand. 

As ligands in metal complexes, the anions derived from these acids display a broad variety of coordination patterns (Scheme 2). Usually, the ligands are described as monodentate, bidentate chelating or bridging, and polydentate. We use here a description based upon the number of connections between the... 

Ion pairing is due to electrostatic forces between ions of opposite charges in a medium of moderate to low relative permittivities. It should be distinguished from complex formation between metal cations and anionic ligands, in which coordinative bonds (donation of an electron pair) takes place. One distingnishing feature is that, contrary to complex formation, the association is nondirectional in space. The association of a cation and an anion to form an ion pair can, however, be represented as an equilibrium reaction by analogy to complex formation with an equilibrium constant A)ass [3,5]. If a is the fraction of the electrolyte that is dissociating into ions and therefore (1 - a) is the fraction that is associated, then... 

In the reaction below a metal ion reacts with n ligand anions h to form an uncharged complex ML. If the ions dissolve only in the aqueous phase, and the metal complex and undissociated acid HL dissolve only in the organic phase, lUPAC allows the reaction to be written in four different ways ... 

A route for designing Gd(HI) complexes whose relaxivity depends on the presence of lactate, is provided by the ability shown by some hexa- or hepta-coordinate chelates to form ternary complexes with a wide array of anionic species (154-161). The interaction between the coordinatively unsatured metal complex and lactate involves the displacement of two water molecules coordinated to Gd(III) ion with the two donor atoms of the substrate, thus leading to a marked decrease in the relaxivity. Lactate is a good ligand for Gd(IH) ion because it can form a stable 5-membered ring by using the hydroxo and carboxylic oxygen donor atoms (Fig. 19). 

Bipyridine is likewise reduced to 4,4 -bipiperidine by sodium and amyl alcohol and by catalytic hydrogenation. " 2,2 -Dimethyl-4,4 -bipyridine is reduced to 2,2 -dimethyl-4,4 -bipiperidine. Electrochemical reduction of 4,4 -bipyridine affords 4,4 -bipiperidine and some partly reduced 4,4 -bipyridines. Further work on the electroreduction of 4,4 -bipyridine has been reported. Reduction of 4,4 -bipyridine by tin and hydrochloric acid " or by controlled catalytic hydrogenation - gives l,2,3,4,5,6-hexahydro-4,4 -bipyridine. 4,4 -Bipyridine is reduced to its 1,4-dihydro derivative by bisdihydropyridyl metal complexes and to its radical anion by alkali metals and related processes. " " The ionization constant of the radical anion has been determined. ... 

This can certainly be extended to other metal sulphides, using other complexes of sulphur (and also selenium). However, the complex and anion of the metal salt need to be chosen so that all the by-products of the pyrolysis reaction are volatile, otherwise the film will be contaminated with the nonvolatile by-products. For example, using cadmium nitrate and thiourea, all the by-products are volatile ... 

The synthesis and structures of transition metal complexes of anionic cyclo-polypnictogen ligands, e.g. cyclo-P5, are discussed in Section 7.2.1 and 11.2. An exciting recent development involves the use of these complexes as building blocks for the construction of novel supramolecular assemblies, including one-dimensional (ID) and two-dimensional (2D) polymers and even soluble spherical fullerene-like aggregates.Complexes that have been used for this... 

The ligand of a metal complex and the anion of the supporting electrolyte compete with each other, as Lewis bases, to interact with the central metal ion. At the same time, the metal ion and the cation of the supporting electrolyte compete with each other, as Lewis acids, to interact with the ligand. Therefore, the behavior of a metal complex is easily influenced by the supporting electrolyte. For example, tris(acetylacetonato)iron(III), Fe(acac)3, in AN forms Fc(acac)i by one-electron reduction. If the supporting electrolyte is LiCl04, however, Li+ picks up acac from Fe(acac)i [35],... 

A. Classes of Water-Soluble Ligands and Metal Complexes 1. Anionic Ligands... 

The chapter by C. J. Swan and D. L. Trimm, which also emphasizes the effect on catalytic activity of the precise form of a metal complex, shows too that, depending on the metal with which it is associated, the same ligand can act either as a catalyst or inhibitor. The model reaction studied was the liquid-phase oxidation of ethanethiol in alkaline solution, catalyzed by various metal complexes. The rate-determining step appears to be the transfer of electrons from the thiyl anion to the metal cation, and it is shown that some kind of coordination between the metal and the thiol must occur as a prerequisite to the electron transfer reaction (8, 9). In systems where thiyl entities replace the original ligands, quantitative yields of disulfide are obtained. Where no such displacement occurs, however, the oxidation rates vary widely for different metal complexes, and the reaction results in the production not only of disulfide but also of overoxidation and hydrolysis products of the disulfide. 

The effects of adding various metal ions and metal complexes on the rate of a model oxidation reaction have been studied in some detail The model reaction chosen—the oxidation of ethanethiol in aqueous alkaline solution in the presence of metal-containing catalysts—involves the transfer of an electron from the thiol anion to the metal The catalytic activity of additives depends upon the solubility of the particular metal complex and varies according to the nature of the ligand attached to the metal ion. In conjunction with different metals, the same ligand can act either as a catalyst or as an inhibitor. The results are discussed in the light of proposed reaction mechanisms. 

A mechanism which is consistent with the various experimental results for olefin formation involves the initial abstraction of the hydrazone proton (103 - 106)82 In this case, however, expulsion of the tosylate anion is associated with the abstraction of a second hydrogen from C-16 instead of hydride attack on the C=N bond (compare 97 - 98 and 106 - 107). Expulsion of nitrogen from the resulting intermediate (107) yields an anion (108) which is most probably stabilized in the form of a metal complex and can be readily decomposed by water to give an olefin (109). This implies that 17-d1-androst-16-ene (104) can be prepared by using deuterium oxide as the sole deuterated reagent.82... 

These observations allow a possible mechanism to be outlined in (59)—(61). In these systems there is a competition between two bases and two acids, the metal complex and the anion of the solvent (NCCH2 or 02NCH2 ) on the one hand, and C02 and H+ on the other. A crucial step in the mech-... 

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