Additions Involving Metal Ions

Certain metal cations. are capable of electrophilic attack on alkenes. Addition is completed when a nucleophile, either from the solvent or the metal ion s coordination sphere, acts as a nucleophile toward the alkene-cation complex  

The best studied of these reactions involve the mercuric ion as the cation/ While the process is feasible for other transition metal cations, the products often go on to react further. The mercuration products are stable and this allows a relatively uncomplicated study of the addition reaction itself. The usual nucleophile is the solvent, either water or an alcohol, but in less nucleophilic solvents other nucleophiles can compete for the complex. The term oxymercuration is used to refer to reaction in which water or alcohols are introduced as the nucleophile. 

A mercurinium ion has both similarities and differences as compared with the intermediates which have been described for other electrophilic additions. The proton which initiates reaction in acid-catalyzed processes is a hard acid and has no unshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron deficient and highly reactive  

The formal species Br which leads to the bromonium ion intermediate is softer and also has unshared electron pairs which can permit a total of four electrons to participate in the bridged bromonium ion intermediate. This would be expected to be more stropgly bridged than would be possible for the proton and the bromonium ion can be represented as having two covalent bonds to bromine. This species is electrophilic but not electron deficient. 

Oxymercuration of simple alkenes is a stereospecific anti addition. This result is in agreement with the involvement of a mercurinium intermediate which is opened by nucleophilic attack. The reactivity of the mercury salt is a function both of the 

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SECTION 6.4. ELECTROPHILIC ADDITIONS INVOLVING METAL IONS... 

The hydroxy a-amino acids l-serine and l-threonine, used as models for the 2-amino-2-deoxy glyconic acids, have been complexed with Ni(II) at 37 °C in aqueous solutions of 0.15M potassium nitrate. Values for the stability constants were obtained from iso-pH titration data which were collected by alternate, small, incremental additions of metal ion and potassium hydroxide being made such that the pH of the solution remained nearly constant. The data were consistent with the predominance of MLn species, along with additional protonated and hydrolyzed complexes. There was no evidence for the involvement of the hydroxyl group in chelation. By the same iterative computations the complexes formed between borate and mannitol have been analyzed, and the stability constants have been calculated. Complexes with mannitohborate stoichiometries of I.T, 1 2, 1 3, and 2 1 were proposed. 

The use of molecular mechanics as an aid in the interpretation of spectroscopic data is outlined in more detail in Chapter 9. One of the most rapidly developing applications of molecular mechanics is the use of the structures to aid in the analysis of multidimensional NMR spectroscopy125,261. This is particularly pertinent to the study of metal-macromolecule interactions where the spectroscopic data often have too low an observation/variable ratio to allow an unequivocal determination of the structure. Therefore, an additional source of structural information is needed. To date, there have been a limited number of studies involving metal ions but this area is likely to become a very important application of structural information from molecular mechanics studies (see also Chapter 9). 

Classical fractionation procedures usually involve precipitation by adjustment of pH, adjustment of salt concentration, addition of organic solvents, or addition of metal ions. The fractionations produced are rather crude, but are generally quick and easy, and the manipulation of pH still remains a popular method today. 

The presence of metal ions in solution can have pronounced effects on the reactivity of phosphate esters. In addition, catalysis of reactions of phosphates by enzymes often involves metal ions. In an earlier review (Kluger, 1978), some of the possible modes of catalysis by metal ions were presented in terms of what was known of available hydrolytic mechanisms. 

The electrochemical response of 1, 2, and 4 films to and Tb + were examined by CV. Figure 6.6 shows a representative CV of films 2 before and after addition of 2.0)xM of Tb. Unlike films involving Fc-crownether, metal interactions with films 1, 2, and 4 do not lead to Ihe appearance of a new redox signal. Instead, the of the Fc/Fc couple is slightly anodicaUy shifted. There is however a drastic decrease in the and, hence, the surface charge with each addition of metal ions which... 

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