Hydrido-cobalt-tetracarbonyl

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

The first catalyst used in hydroformylation was cobalt. Under hydroformylation conditions at high pressure of carbon monoxide and hydrogen, a hydrido-cobalt-tetracarbonyl complex (HCo(CO)4) is formed from precursors like cobalt acetate (Fig. 4). This complex is commonly accepted as the catalytic active species in the cobalt-catalyzed hydroformylation entering the reaction cycle according to Heck and Breslow (1960) (Fig. 5) [20-23]. 

The hydrido-cobalt-tetracarbonyl complex (I) undergoes a CO-dissocia-tion reaction to form the 16-electron species HCo(CO)3 (II). This structure forms a 7r-complex (III) with the substrate and is a possible explanation for the formation of further (C = C)-double bond isomers of the substrate. In the... 

In the next step of the reaction cycle, the carbon monoxide is inserted into the carbon-cobalt bond. At this time, the subsequent aldehyde can be considered as preformed. This step leads to the 16 electron species (VI). Once again, carbon monoxide is associated to end up in the 18 electron species (VII). In the last step of the reaction cycle, hydrogen is added to release the catalyti-cally active hydrido-cobalt-tetracarbonyl complex (I). Likewise, the aldehyde is formed by a final reductive elimination step. 

The catalyst precursor is the 18-electron hydrido cobalt tetracarbonyl complex A, which dissociates a CO ligand to give the 16-electron active catalyst B. The next step is the coordination of alkene to give the 18-electron it complex C. This is followed by rapid insertion of the alkene into the metaTTiydrogen bond by hydride migration to form the cobalt alkyl complex D. The next step is addition of CO from the gas phase to afford the 18-electron tetracarbonyl complex E, which undergoes CO insertion to give the 16-electron acyl complex F. This is followed by oxidative addition of H2 to the Co acyl complex to form the 18-electron Co dihydrido complex G. 

The field of both a- and w-allyl complexes was brought into focus following studies by Jonassen and co-workers on the complex C4H7Co(CO)3, which they prepared by treating hydrido-cobalt tetracarbonyl with butadiene. An early structural proposal for this complex was shown in (V) (59). 

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