Alkylcobalt coordination

The n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

Their view that cobalt hydrotricarbonyl, instead of die hydrotetra-carbonyl, is the reactive species is based on evidence that the formation of alkylcobalt tetracarbonyl is inhibited by carbon monoxide more fundamentally, initial complexing with olefin would presumably require the participation of a coordinately unsaturated carbonyl. 

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]. 

Alkylcobalts form in stoichiometric reactions of pentacyanocobalt(III) hydride and alkenes. This reaction occurs both for halogenated alkenes such as tetrafluoroethylene and for alkenes that contain other electron-withdrawing groups such as carbonyls, nitriles and arenes as substituents (see Table 6) . The addition is regiospedfic, forming the more substituted alkylcobalt. Prior coordination of alkene to cobalt to form an alkene(hydrido)cobalt complex, an intermediate in hydrometalation reactions, is not important. This reaction is a radical process however, by NMR, additions of [HCo(CN)5 ] " to diastereomeric alkenes such as fumaric and maleic add salts lead to a cr-alkylcobalt by stereospecific cis addition of Co and H to the double bond . The overall reduction is not stereospecific. (r-Alkylcobalt bond formation proceeds by either a concerted addition or a rapid collapse of a radical cage. 

Coordination of an alkene to the coordinatively unsaturated HCo(CO)2L 2 followed by insertion leads to the formation of an alkylcobalt intermediate 3. 

The coordinated carbonyl groups of the alkylcobalt and acylcobalt tetracarbonyls may be replaced by other ligands. When alkylcobalt tetracarbonyls react with ligands, they generally form acylcobalt tricarbonyl derivatives. 

Indirect evidence of the existence of coordinatively unsaturated allgrlcobalt tricarbonyl species has been obtained from the kinetics of substitution with PlCeHsis (168) and with (168), (180) and from the kinetics of the reductive cleavage by H2 and HCo(CO)4 (168) and by HMn(CO)5 (215) of alkylcobalt... 

The stability constants for the coordination of pyridine or substituted pyridines to various alkylcobalt porphyrin systems have been reported. This study has been done in order to observe the m-influence of hydroporphyrin macrocycles on axial position and alkyl exchange reactions. The alkyl exchange reactions of organocobalt(lll) porphyrins with a cobalt(ll) complex of a distinguishable porphyrin or tetrapyrrolc have been studied. The equilibrium constants for the alkyl transfer have been reported. The exchange of the axial ligand is reversible and it follows a bimolecular mechanism. Thermodynamic and activation parameters for homolytic Co-C bond dissociation have been obtained on the (TAP)CoC(CH3)2CN complex (TAP = tetraanisylporphirinato). ... 

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