Methylcobalt tetracarbonyl

The first alkylcobalt tetracarbonyl, and indeed the first simple alkylcobalt compound of any type, was prepared by Hieber and his co-workers (20). The compound prepared, methylcobalt tetracarbonyl, was obtained in 2%... 

Acylcobalt carbonyl derivatives are cleaved by sodium methoxide to esters and sodium salts of the corresponding carbonyl anions. By means of this reaction, acetyl[bis(trimethylolpropane phosphite)]cobalt dicarbonyl has been converted into sodium [bis(trimethylolpropane phosphite)]cobalt dicarbonyl. The latter compound is readily alkylated by methyl iodide to form the methylcobalt derivative and this compound in turn reacts with another mole of the phosphite ester, in the same way that methylcobalt tetracarbonyl does, to form acetyl[tris(trimethylolpropane phosphite)]-cobalt carbonyl ... 

The addition of acylcobalt carbonyls to conjugated dienes is a very general reaction (14, 15). Some information on the reactivity of differently substituted double bonds in the diene system can be obtained from the structures of the l-acylmethyl-7r-allylcobalt tricarbonyls formed from them. The acyl group is reasonably assumed to add to the least-hindered and most reactive double bond. Isoprene reacts with methylcobalt tetracarbonyl to form l-acetylmethyl-2-methyl-7r-allylcobalt tricarbonyl. 

Thus, the monosubstituted double bond is more reactive than the di-substituted. trawi-Piperylene and methylcobalt tetracarbonyl form 1-acetylmethyl-4-methyl-77-allylcobalt tricarbonyl. 

By the addition of a phosphine to an alkylcobalt tetracarbonyl, the phosphine-substituted acylcobalt carbonyl can be prepared (182,183). In the case of (methoxycarbonyl)methylcobalt tetracarbonyl as a model compound, some details of the reaction have been revealed. Adding a phosphine to this complex, the kinetically controlled formation of a phosphine-substituted acylcobalt carbonyl is observed that can be converted to the thermodynamically more stable phosphine -substituted derivative. 

Acetylcobalt tricarbonyl obtained by irradiation in low temperature matrices has been characterized by infrared spectroscopy (220). The anomalously low intensity band at 1685.9 cm was assigned to the C-0 stretch of the acetyl group, which is believed to be coordinated in a if-mode. The observation of the spectrum of acetylcobalt tricarbonyl is complicated by the fact that bands due to the parent acetylcobalt tetracarbonyl and the product methylcobalt tetracarbonyl are observed in the same region of the spectrum (220). 

Such aspects of metal carbonyl structure may be explained by consideration of the coordination number of the central metal atom as an important factor in determining the stability of metal carbonyls. As is the case with other transition metal derivatives such as the ammines, octahedral hexa-coordinate metal carbonyl derivatives seem to be especially favored. Thus, hexacoordinate chromium hexacarbonyl is obviously more stable and less reactive than pentacoordinate iron pentacarbonyl or tetracoordinate nickel tetracarbonyl. Moreover, hexacoordinate methylmanganese pentacarbonyl is indefinitely stable at room temperature (93) whereas pentacoordinate methylcobalt tetracarbonyl (55) rapidly decomposes at room temperature and heptacoordinate methylvanadium hexacarbonyl has never been reported, despite the availability of obvious starting materials for its preparation. 

The volatile methylcobalt tetracarbonyl is thermally very unstable, m.p. —44°, and it decomposes at —35° [60]. It is readily oxidiz. The acylation reactions and the role of alkylcobalt carbonyl complexes in catalytic hydroformylation reactions are discussed on p. 334 and in chapter 9 respectively. 

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