Hydridocobalt carbonyls

Simple cobalt carbonyl CO (CO)t( or, rather, hydridocobalt carbonyl HCo(CO)4. complexes. 

The possible sources of isomeric aldehyde formation include olefin isomerization, regioselectivity of the addition of the hydridocobalt carbonyl to the olefin, isomerization of the alkylcobalt carbonyl, and isomerization of the acylco-balt carbonyl species. There is no evidence for an isomerization of the alkylcobalt carbonyl species under the conditions of industrial oxo synthesis (high pressure) [96]. In contrast, the isomerization of a coordinated olefin is well known and a plethora of studies have proven this behavior [4]. 

The selective reaction of the hydridocobalt carbonyl with the olefin via Mar-kovnikov and anti-Markovnikov addition gives rise to the branched and linear alkylcobalt carbonyl isomers. It is believed that the sterically less demanding nature of HCo(CO)3 favors the formation of the branched isomer, whereas HCo(CO)4 generates predominantly the linear isomer. This is in accordance with the increased selectivity observed at higher carbon monoxide partial pressures. As HCo(CO)4 is the less reactive catalyst, the catalytic activity drops at the same time. 

According to the mechanism described above, phosphine-modified cobalt catalysts Co2(CO)6(L)2 behave in the same way. It is generally accepted that the selective antCMarkovnikov addition of the hydridocobalt carbonyl to the olefin forced by steric hindrance determines the n/i ratio. 

Reductive Cleavage of Acylcobalt Carbonyls. The product-forming step in hydroformylation is the reductive cleavage of the acyl complex either by dihydrogen or by a hydridocobalt carbonyl. 

The reaction with dihydrogen results in the formation of substituted hydridocobalt-carbonyl species. 

The treatment of a cobalt(II) salt with synthesis gas generates sequentially Co2(CO)8 then HCo(CO>4. This catalyst is generated only at 120-140 C for the carbonylation to proceed smoothly 200-300 bar is required to stabilize the catalyst. If the hydridocobalt catalyst is prepared separately and then introduced into the reaction, temperatures as low as 90 C can be used for the hydrocarbonylation. An important consideration in industrial reactions is the normal to branched nib ratio to give the desired straight chain aldehyde, the hydridocobalt catalyst providing an nib ratio of -4 in the hydroformylation of propene under the lower temperature conditions. This catalyst will stoichiometrically hydroformylate 1-alkenes under ambient conditions. 

The diethyl ester of phenylphosphonous acid (diethoxyphenyl-phosphine) provides an easy pathway to relatively stable telrakis complexes of zero- and low-valent transition metals.1,2 Anhydrous metal halides serve as the metal source for the complexes, avoiding the necessity of inconvenient starting materials such as nickel carbonyl. The nickel(O) complex is formed by reaction with the phosphonite in ethanol with the addition of sodium tetrahydroborate, relatively stable dihydridoiron(l I) and hydridocobalt(I) complexes are obtained. 

The mass of this work lies beyond the scope of this review, but a few prominent points may be cited. In most cases, treatment of either an alkyl or an acyl tetracarbonyl cobalt complex with triphenylphosphine gives only the acyl derivative, [Co(COR)(CO)3P(C6H5)3] (304, 306). The reactions of acyl, allyl, and hydridocobalt tetracarbonyl or acetylenedicobalt hexa-carbonyl complexes with phosphines are first order in the cobalt complex, so a dissociation mechanism prevails (64, 65, 66). However, the reaction between [Co(CO)3NO] and P(C6H5)3 is second order (66). 

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