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Concept
Dicobalt octacarbonyl
Summary
Dicobalt octacarbonyl is an organocobalt compound with composition . 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 parent member of a family of hydroformylation catalysts. Each molecule consists of two cobalt atoms bound to eight carbon monoxide ligands, although multiple structural isomers are known. Some of the carbonyl ligands are labile.
Dicobalt octacarbonyl an orange-colored, pyrophoric solid. It is synthesised by the high pressure carbonylation of cobalt(II) salts:
The preparation is often carried out in the presence of cyanide, converting the cobalt(II) salt into a pentacyanocobaltate(II) complex that reacts with carbon monoxide to yield . Acidification produces cobalt tetracarbonyl hydride, , which degrades near room temperature to dicobalt octacarbonyl and hydrogen. It can also be prepared by heating cobalt metal to above 250 °C in a stream of carbon monoxide gas at about 200 to 300 atm:
It exist as a mixture of rapidly interconverting isomers. In solution, there are two isomers known that rapidly interconvert:
The major isomer (on the left in the above equilibrium process) contains two bridging carbonyl ligands linking the cobalt centres and six terminal carbonyl ligands, three on each metal. It can be summarised by the formula and has C2v symmetry. This structure resembles diiron nonacarbonyl () but with one fewer bridging carbonyl. The Co–Co distance is 2.52 Å, and the Co–COterminal and Co–CObridge distances are 1.80 and 1.90 Å, respectively. Analysis of the bonding suggests the absence of a direct cobalt–cobalt bond.
The minor isomer has no bridging carbonyl ligands, but instead has a direct bond between the cobalt centres and eight terminal carbonyl ligands, four on each metal atom. It can be summarised by the formula and has D4d symmetry. It features an unbridged cobalt–cobalt bond that is 2.70 Å in length in the solid structure when crystallized together with C60.
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