page_title: Dicobalt octacarbonyl
section_title: Hydroformylation
hierarchical_section_title: Dicobalt octacarbonyl / Reactions / Hydroformylation
caption_attribution_description: English: chemical structure of HCCo3(CO)9
context_page_description: Dicobalt octacarbonyl is the organometallic compound with composition Co₂(CO)₈. This metal carbonyl is used as a reagent and catalyst in organometallic chemistry and organic synthesis, and is central to much known organocobalt chemistry. It is the precursor to a hydroformylation catalyst, cobalt tetracarbonyl hydride. Each molecule consists of two cobalt atoms bound to eight carbon monoxide ligands, though multiple distinct structural arrangements are known. Some of the carbonyl ligands are highly labile. The compound is highly reactive towards alkynes, and is sometimes used as an alkyne protecting group. As the cobalt-alkyne complex, it plays a role in promoting both the Nicholas reaction and the Pauson–Khand reaction.
context_section_description: Hydrogenation of Co₂(CO)₈ produces cobalt tetracarbonyl hydride, [HCo(CO)₄]:
  Co₂(CO)₈  +  H₂  →  2 HCo(CO)₄
  This hydride is used as a catalyst for hydroformylation – the conversion of a terminal alkene, RCH=CH₂, to an aldehyde, RCH₂CH₂CHO.  The catalytic cycle for this hydroformylation is shown in the diagram.
  Reduction of Co₂(CO)₈ with sodium amalgam gives the conjugate base of HCo(CO)₄.  This salt yields the hydride on acidification, providing an alternative synthetic pathway to that species.  Salts of this form are also intermediates in the cyanide synthesis pathway for dicobalt octacarbonyl.
  Co₂(CO)₈  +  2 Na  →  2 NaCo(CO)₄
  NaCo(CO)₄  +  H⁺  →  HCo(CO)₄  +  Na⁺
  The CO ligands can be replaced with tertiary phosphine ligands to give Co₂(CO)8−ₓ(PR₃)ₓ.  These bulky derivatives are more selective catalysts for hydroformylation reactions. "Hard" Lewis bases, e.g. pyridine, cause disproportionation:
  12 C₅H₅N  +  3 Co₂(CO)₈  →   2 [Co(C₅H₅N)₆][Co(CO)₄]₂  +  8 CO
caption: Methylidynetricobaltnonacarbonyl, HCCo3(CO)9, an organocobalt cluster compound structurally related to tetracobalt dodecacarbonyl
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