Cobalt complex compounds structure

Finally, a history of organometallic chemistry would be incomplete without mention of what surely is the oldest known organometallic compound, vitamin B12 coenzyme. This naturally occurring cobalt complex, whose structure is illustrated in Figure 13-6, contains a cobalt-carbon n bond. It is a cofactor in a number of enzymes that catalyze 1,2 shifts in biochemical systems ... 

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... 

The seminal studies on these complex compounds were conducted by Alfred Werner in an intensive period of work at the turn of the century. A typical example of the problems that Werner addressed lies in the various compounds which can be obtained containing cobalt, ammonia and chlorine. Stable and chemically distinct materials with formulations Co(NH3) Cl3 (n = 4,5 or 6) can be isolated. The concepts of valency and three-dimensional structure in carbon chemistry were being developed at that time, but it was apparent that the same rules could not apply to... 

Varying ratios of the ligands 7V-(2-thiophenyl)-2,5-dimethylpyrrole and V-methylimidazole were used to form tetrahedral zinc complexes with S4, S3N, and S2N2 coordination spheres. X-ray structural analyses and IR spectra were recorded for all compounds and the relevance to zinc finger proteins was discussed. The comparison to cobalt and cadmium structures showed only minor differences, supporting the theory that changes on substituting these metals into zinc proteins would be minor.538... 

It would be interesting to know whether this enzyme system works with cobalt only when the latter is held in the vitamin B12 structure, or whether it is possible to produce cobalt-carbon compounds from other cobalt complexes as well. 

It is apparent from most of the examples previously described that the most common formal oxidation state found for the Group 14 element is E(IV) (E = Ge, Sn, Pb). Relatively few examples of divalent germanium, tin, or lead complexes have been described, and of these, many are not well characterized. Cobalt-containing compounds are no exception in this regard and there appears to be only one report in the literature that describes a species of this type, viz. [Ge Co(CO)4 2], 67, although the precise structure of this complex is unknown (77). Two main synthetic routes are described, Eqs. (4) and (S), the starting complex in the latter reaction being... 

Despite the incontrovertible evidence regarding the structure of copper and nickel complexes of o-hydroxydiarylazo compounds, confusion remained with regard to their cobalt complexes. Thus some workers13a b>14,16 reported the isolation of complexes having 2 1 stoichiometry whilst others5 17 reported 3 1 stoichiometry. The oxidation state of the cobalt was also in dispute. The situation was clarified18 when more modem techniques were employed to study the reaction of l-phenylazo-2-naphthol and related compounds with various cobalt salts and complexes. The results of this work are summarized in Scheme 1. 

There is no doubt that Schetty s rules have a certain validity but great care must be exercised in their application since structural features of azo compounds other than their coordinating system can dramatically affect the geometry of the derived 1 2 chromium and cobalt complexes. 

Extension of the NMR studies described in Section 58.2.3.3(i)(b) to the compounds (136 R = N02 and R = Bu ) led the authors to conclude that one Na- and two No-coordinated cobalt complexes were produced. These were assigned the partial structures (137), (138) and (139), which involve coordination of the ligand in its azo form and in the two alternative hydrazone forms. In general (Section 58.2.3.2), azophenols exist exclusively in the azo form and serious doubts must exist regarding structure (139). Similarly, in view of the reported equilibration between the various isomers in solution, assignment of discrete tautomeric structures must be open to question. As-... 

The iron compound readily sublimes and yields well-formed, black lustrous crystals. The cobalt complex will also readily sublime, but dependent upon the temperature at which the crystals are formed, they can be either black or brown in color. The crystal structures of both the cobalt and iron complexes have been determined.3 The nickel complex sublimes only in small amounts with difficulty. All three complexes are unstable to air and water, and the nickel complex readily undergoes thermal decomposition above 100°C. All three compounds will also readily form complexes with a variety of donor ligands such as tertiary arsines or phosphines. The nickel compound usually forms 2 1 adducts such as [(C6HS )3P]2Ni(NO)I, while the iron and cobalt complexes often undergo disproportionation.5... 

Another structural variation operated in these compounds is the substitution at the angular 1,19-positions with methyl groups in natural corrinoids only the 1 postion is substituted. Such alteration is necessary on one hand because the cobalt complex of 1-methyl-octadehydrocorrin is unstable and, on the other hand, the related [Co(A2TDC)]+ is prepared by reduction of [Co(A2ODC)] + [2]. 

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