Cobalt complexes reactivity with

The electrochemical synthesis of atylzinc or heteroarylzinc species bearing electron-donating or -withdrawing groups can thus be carried out using nitJcel [14] or cobalt [15] as catalyst under mild conditions. Subsequently, the fourth part of this chapter will deal with formations of these species prepared by electrochemical reaction catalyzed by nickel or cobalt complexes, along with their reactivity. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

With sp bond angles calculated to be around 162°, macrocycle 131 would be highly strained and was therefore expected to be quite reactive [79]. The octa-cobalt complex 132, on the other hand, should be readily isolable. Indeed, 132 was prepared easily from 133 in five steps, and was isolated as stable, deep maroon crystals (Scheme 30). All spectroscopic data supported formation of the strain-free dimeric structure. Unfortunately, all attempts to liberate 132 from the cobalt units led only to insoluble materials. Diederich et al. observed similar problems when trying to prepare the cyclocarbons [5c]. Whether the failure to prepare these two classes of macrocycles is due to the extreme reactivity of the distorted polyyne moiety or to the lack of a viable synthetic route is not certain. Thus, isolation and characterization of smaller bent hexatriyne- and octatetrayne-containing systems is an important goal that should help answer these questions. 

Anionic complexes of iron and cobalt rapidly react with permethylated a,a>-dihalopolysilanes to give disubstituted polysilanes [1,2]. Because of the low nucleophilicity of pure carbonyl-cobaltate, we substituted one of the CO ligands with PPh3, whereupon the reactivity increased dramatically. 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

The y-lactone-type cobalt complexes described in Section III,A have further been studied with regard to spectroscopy (317), reactivity... 

Both 2 1 chromium and cobalt complex azo dyestuffs have little or no affinity for cellulosic fibres and until the early 1960s their use was restricted to wool and nylon. With the introduction by ICI of their Procion range of fibre-reactive dyes, however, their use was extended to cellulosic fibres on which they give prints having excellent fastness to light and wet treatments. Before that time the development of metal complex dyes for cellulose had followed a similar pattern to that of the development of such dyes for wool but, in this case, the most important metal was copper. Early work in this field has been reviewed by several authors.1 The after-treatment of dyeings on cotton obtained from dyestuffs such as (11) with copper salts was used for many years to improve fastness... 

The porphyrin-cobalt complex gives rise to the cation radical with charge spin localization at the nitrogen atom of the porphyrin ring. The cation radical thus formed acquires enhanced reactivity and can add tolane (Kochi 1986), Scheme 1-52. The main point... 

The second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

Modified carbonylcobalt complexes can catalyze the PKR. One or more CO can be substituted by phosphines, and these can be immobilized in resins thus giving anchored cobalt complexes (27), that were able to catalyze the reaction of 28 giving 29 with good yield and minor amounts of 30 (Scheme 10) [76]. Other cobalt metal clusters like Co4(CO)i2 [77] or methyl-idynetricobalt nonacarbonyl [78] have exhibited high reactivity in the catalytic PKR. 

The coordination chemistry of cobalt can be summed up in three words synthesis, structure and reactivity, with synthesis being the most important because it comes first. This chapter concentrates on synthetic aspects and its aim is to provide the reader with starting points to the preparation of cobalt complexes in addition to surveying their chemistry. 

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