Dicobalt octacarbonyl structure

Fig. 15.6 Alternative structures for dimanganese decacarbonyl and dicobalt octacarbonyl. Structure (a) is unknown, but there is infrared evidence for the e,xistence of (b) in solution.

The dark red crystalline compound C ELCosOa formed by treating dicobalt octacarbonyl with acetylene, carbon monoxide, hydrogen, and isopropyl alcohol (57) may have a similar structure, i.e., (LXVIII R = allyl and one CO ligand replaced by the C=C bond of the allyl group). 

IV. Properties, Structure, and Preparation of Dicobalt Octacarbonyl and Cobalt... 

In the carbonyls, each molecule of carbon monoxide donates two electrons to the central atom. Cobalt has 27 extra nuclear electrons, and if two electrons are contributed by each of four carbon monoxide molecules, the cobalt would have an E.A.N. of 35. One more electron is needed to attain the rare gas structure of krypton and this is secured by the sharing of one electron pair between two cobalt monomers. The existence of a metal to metal bond in dicobalt octacarbonyl has been postulated by Ewens (34) and cryoscopic measurements have established without doubt the dimeric structure [Co(CO)4]2 for dicobalt octacarbonyl. 

In addition, its proton magnetic resonance spectrum unambiguously supported the 5-ketose structure (69). Rearrangements of epoxides to ketones when dicobalt octacarbonyl is used as the catalyst at temperatures above 100 , or when cobalt hydrocarbonyl is used at lower temperatures, are well known. By applying the technique of double irradiation to a sample of (68), the main component was shown to possess structure (68). Presumably, the free aldehyde group of the hydroformylation product immediately cyclized with the free hydroxyl group on C-3 to give the tricyclic structure (68). A third component (68a) (isolated in less than 5 % yield) was undoubtedly formed by subsequent reduction of the dialdose derivative (68). 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Fig. 6. Dicobalt octacarbonyl (a) structure (after (39)) (b) bent bond description (c) straight bond description...

An idea of the structure of I may be gained by examining the complex obtained from the reaction of acetylene with dicobalt octacarbonyl (7, 8). The stoichiometry of this reaction indicates that the acetylene complex, III, is formed in the following manner ... 

These facts may be explained in the following manner, using 1-pentene and 2-pentene as examples. Because of steric hindrance, 2-pentene reacts with dicobalt octacarbonyl to form a complex more slowly than 1-pentene, and this accounts for the differences in rates observed with these olefins. It appears that the energy required for the rearrangement of the complex subsequent to its initial formation is small we may therefore conclude that essentially the same complex is obtained from both terminal and internal olefiins. The structure given for the olefin-carbonyl complex I probably represents the complex as it initially forms from either a terminal or internal olefin. It is not possible at present to write an adequate structure... 

Cyclobutadiene-metal complexes have also been suggested as intermediates in a number of other reactions, notably in the formation of benzenes by trimerization of acetylenes 2, 18, 57, 90) and also in other reactions. Conclusive evidence is still lacking, but the inertness of the known cyclobutadiene complexes towards acetylenes in Diels-Alder type reactions makes this rather unlikely. Intermediates with open-chain structures such as (XCVI) appear more attractive. Arnett and Bollinger (2) have isolated by-products from the dicobalt-octacarbonyl-catalyzed trimerization of diisopropylacetylene which are very similar to some, e.g., (LI), obtained in the thermal decomposition of tetramethylcyclobutadienenickel chloride complexes (29). Again, here, however, the evidence is by no means conclusive and a variety of intermediates other than a cyclobutadiene-metal complex can be postulated to explain the observed products however, see also the Appendix. 

The structure of vanadium carbonyl is again different from those of dimanganese decacarbonyl or dicobalt octacarbonyl. Despite an early report... 

Dicobalt octacarbonyl is known to exist in two isomeric forms, as shown in Figure 17. A bridged structure of this molecule is observed in the sohd state, as weU as a solution state at a very low temperature. A non-bridged structure predominates in a solution at temperatures above ambience. 

Figure 17 Structure of dicobalt octacarbonyl (without bridge and with bridge).

Discuss the preparation, properties and structure of dicobalt octacarbonyl. 

The reaction of l,r-dialkynylferrocene [(Ti5-C5H4CCR)2Fe] (R = Ph, SiMe3, Me, ferrocenyl) with excess dicobalt octacarbonyl afforded [ (T 5-C5H4CCR)2Fe Co2(CO)6)2l. structurally characterised for R = Ph, in which a Co2(CO)6) group coordinates to each of the alkyne moieties. 

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