Dicobalt octacarbonyl

Dicobalt octacarbonyl an orange-colored, pyrophoric solid. It is synthesised by the high pressure carbonylation of cobalt(II) salts:{{cite book |doi=10.1002/9780470132333.ch76|title=Dicobalt Octacarbonyl, Cobalt Nitrosyl Tricarbonyl, and Cobalt Tetracarbonyl Hydride |series=Inorganic Syntheses |year=1946 |last1=Gilmont |first1=Paul |last2=Blanchard |first2=Arthur A. |pages=238–243 |isbn=9780470132333|volume =2 }}

:{{chem2|2 (CH3COO)2Co + 8 CO + 2 H2 → Co2(CO)8 + 4 CH3COOH}}

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 {{chem2|K[Co(CO)4]}}. Acidification produces cobalt tetracarbonyl hydride, {{chem2|HCo(CO)4}}, which degrades near room temperature to dicobalt octacarbonyl and hydrogen.{{cite book|chapter = Hydrogenation of Organic Compounds with Synthesis Gas|first = Milton|last = Orchin|title = Advances in Catalysis|year = 1953|volume = 5|pages = 385–415|chapter-url = https://books.google.com/books?id=f_Oiij4V98cC&pg=PA409|publisher = Academic Press|isbn = 9780080565095}} 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:

:{{chem2|2 Co + 8 CO → Co2(CO)8}}

It exists as a mixture of rapidly interconverting isomers. In solution, there are two isomers known that rapidly interconvert:{{cite journal|first1 = Ray L.|last1 = Sweany|first2 = Theodore L.|last2 = Brown|authorlink2 = Theodore L. Brown|title = Infrared spectra of matrix-isolated dicobalt octacarbonyl. Evidence for the third isomer|journal = Inorganic Chemistry|year = 1977|volume = 16|issue = 2|pages = 415–421|doi = 10.1021/ic50168a037}}

:File:Co2(CO)8NoCo-Co.png

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 {{chem2|(CO)3Co(μ\-CO)2Co(CO)3}} and has C_2v symmetry. This structure resembles diiron nonacarbonyl ({{chem2|Fe2(CO)9}}) but with one fewer bridging carbonyl. The Co–Co distance is 2.52 Å, and the Co–CO_terminal and Co–CO_bridge distances are 1.80 and 1.90 Å, respectively.{{cite journal|first1 = G. Gardner|last1 = Sumner|first2 = Harold P.|last2 = Klug|first3 = Leroy E.|last3 = Alexander|title = The crystal structure of dicobalt octacarbonyl|journal = Acta Crystallographica|year = 1964|volume = 17|issue = 6|pages = 732–742|doi = 10.1107/S0365110X64001803|doi-access = free}} Analysis of the bonding suggests the absence of a direct cobalt–cobalt bond.{{cite journal|first1 = Jennifer C.|last1 = Green|first2 = Malcolm L. H.|last2 = Green|authorlink2 = Malcolm Green (chemist)|first3 = Gerard|last3 = Parkin|authorlink3 = Gerard Parkin|title = The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds|journal = Chemical Communications|year = 2012|volume = 2012|issue = 94|pages = 11481–11503|doi = 10.1039/c2cc35304k|pmid = 23047247}}

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 {{chem2|(CO)4Co\–Co(CO)4}} and has D_4d symmetry. It features an unbridged cobalt–cobalt bond that is 2.70 Å in length in the solid structure when crystallized together with C₆₀.{{cite journal|first1 = Thelma Y.|last1 = Garcia|first2 = James C.|last2 = Fettinger|first3 = Marilyn M.|last3 = Olmstead|first4 = Alan L.|last4 = Balch|title = Splendid symmetry: Crystallization of an unbridged isomer of Co₂(CO)₈ in Co₂(CO)₈^·C₆₀|journal = Chemical Communications|year = 2009|volume = 2009|issue = 46|pages = 7143–7145|doi = 10.1039/b915083h|pmid = 19921010}}

{{Gallery

| title = Isomers of dicobalt octacarbonyl

| File:Dicobalt-octacarbonyl-C2v-bridged-from-xtal-1983-3D-balls-A.png

| Bridged C_2v isomer

| File:Dicobalt-octacarbonyl-D3d-non-bridged-from-C60-xtal-2009-3D-balls.png

| non-bridged D_3d isomer

| File:Dicobalt-octacarbonyl-D2d-non-bridged-3D-balls.png

| nonbridged D_2d isomer

}}

Dicobalt octacarbonyl is reductively cleaved by alkali metals and related reagents, such as sodium amalgam. The resulting sodium tetracarbonylcobaltate protonates to give tetracarbonyl cobalt hydride:{{cite encyclopedia|last1 = Donaldson|first1 = John Dallas|last2 = Beyersmann|first2 = Detmar|year = 2005|title = Cobalt and Cobalt Compounds|encyclopedia = Ullmann's Encyclopedia of Industrial Chemistry|publisher = Wiley-VCH|doi = 10.1002/14356007.a07_281.pub2|isbn = 3527306730}}

:{{chem2|Co2(CO)8 + 2 Na → 2 Na[Co(CO)4]}}

:{{chem2|Na[Co(CO)4] + H+ → H[Co(CO)4] + Na+}}

Salts of this form are also intermediates in the cyanide synthesis pathway for dicobalt octacarbonyl.

Halogens and related reagents cleave the Co–Co bond to give pentacoordinated halotetracarbonyls:

:{{chem2|Co2(CO)8 + Br2 → 2 Br[Co(CO)4]}}

Cobalt tricarbonyl nitrosyl is produced by treatment of dicobalt octacarbonyl with nitric oxide:

:{{chem2|Co2(CO)8 + 2 NO → 2 Co(CO)3NO + 2 CO}}

The Nicholas reaction is a substitution reaction whereby an alkoxy group located on the α-carbon of an alkyne is replaced by another nucleophile. The alkyne reacts first with dicobalt octacarbonyl, from which is generated a stabilized propargylic cation that reacts with the incoming nucleophile and the product then forms by oxidative demetallation.{{cite journal|last = Nicholas|first = Kenneth M.|title = Chemistry and synthetic utility of cobalt-complexed propargyl cations|journal = Acc. Chem. Res.|year = 1987|volume = 20|issue = 6|pages = 207–214|type = Review|doi = 10.1021/ar00138a001}}{{cite journal|last = Teobald|first = Barry J.|journal = Tetrahedron|year = 2002|title = The Nicholas reaction: The use of dicobalt hexacarbonyl-stabilised propargylic cations in synthesis|volume = 58|issue = 21|pages = 4133–4170|type = Review|doi = 10.1016/S0040-4020(02)00315-0}}

Image:Nicholas Reaction Scheme.png

The Pauson–Khand reaction,{{cite journal|first1 = P. L.|last1 = Pauson|authorlink1 = Peter Pauson|first2 = I. U.|last2 = Khand|journal = Ann. N. Y. Acad. Sci.|year = 1977|title = Uses of Cobalt-Carbonyl Acetylene Complexes in Organic Synthesis|volume = 295|issue = 1|pages = 2–14|doi = 10.1111/j.1749-6632.1977.tb41819.x|bibcode = 1977NYASA.295....2P|s2cid = 84203764}} in which an alkyne, an alkene, and carbon monoxide cyclize to give a cyclopentenone, can be catalyzed by {{chem2|Co2(CO)8}},{{cite journal|title = The Pauson–Khand reaction, a powerful synthetic tool for the synthesis of complex molecules|year = 2004|first1 = Jaime|last1 = Blanco-Urgoiti|first2 = Loreto|last2 = Añorbe|first3 = Leticia|last3 = Pérez-Serrano|first4 = Gema|last4 = Domínguez|first5 = Javier|last5 = Pérez-Castells|journal = Chem. Soc. Rev.|volume = 33|issue = 1|pages = 32–42|doi = 10.1039/b300976a|pmid = 14737507}} though newer methods that are more efficient have since been developed:{{cite journal|last = Schore|first = Neil E.|authorlink = Neil E. Schore|title = The Pauson–Khand Cycloaddition Reaction for Synthesis of Cyclopentenones|journal = Org. React.|year = 1991|volume = 40|pages = 1–90|doi = 10.1002/0471264180.or040.01|isbn = 0471264180}}{{cite journal|first1 = Susan E.|last1 = Gibson|first2 = Andrea|last2 = Stevenazzi|title = The Pauson–Khand Reaction: The Catalytic Age Is Here!|journal = Angew. Chem. Int. Ed.|year = 2003|volume = 42|issue = 16|pages = 1800–1810|doi = 10.1002/anie.200200547|pmid = 12722067}}

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{{chem2|Co2(CO)8}} reacts with alkynes to form a stable covalent complex, which is useful as a protective group for the alkyne. This complex itself can also be used in the Pauson–Khand reaction.

Intramolecular Pauson–Khand reactions, where the starting material contains both the alkene and alkyne moieties, are possible. In the asymmetric synthesis of the Lycopodium alkaloid huperzine-Q, Takayama and co-workers used an intramolecular Pauson–Khand reaction to cyclise an enyne containing a tert-butyldiphenylsilyl (TBDPS) protected primary alcohol.{{cite journal|title = Asymmetric Total Synthesis of a Pentacyclic Lycopodium Alkaloid: Huperzine-Q|first1 = Atsushi|last1 = Nakayama|first2 = Noriyuki|last2 = Kogure|first3 = Mariko|last3 = Kitajima|first4 = Hiromitsu|last4 = Takayama|year = 2011|journal = Angew. Chem. Int. Ed.|doi = 10.1002/anie.201103550|volume = 50|issue = 35|pages = 8025–8028|pmid = 21751323}} The preparation of the cyclic siloxane moiety immediately prior to the introduction of the dicobalt octacarbonyl ensures that the product is formed with the desired conformation.{{cite book|chapter = Dicobalt Octacarbonyl|year = 2016|chapter-url = https://books.google.com/books?id=AO3bCwAAQBAJ&pg=PA251|pages = 251–252|title = Fiesers' Reagents for Organic Synthesis|volume = 28|first = Tse-Lok|last = Ho|publisher = John Wiley & Sons|isbn = 9781118942819}}

File:Pauson-Khand reaction in synthesis of huperzine-Q.jpg

Dicobalt octacarbonyl can catalyze alkyne trimerisation of diphenylacetylene and its derivatives to hexaphenylbenzenes.{{cite journal |last1=Vij |first1=V. |last2=Bhalla |first2=V. |last3=Kumar |first3=M. |date=8 August 2016 |title=Hexaarylbenzene: Evolution of Properties and Applications of Multitalented Scaffold |journal=Chemical Reviews |volume=116 |issue=16 |pages=9565–9627 |doi=10.1021/acs.chemrev.6b00144}} Symmetrical diphenylacetylenes form 6-substituted hexaphenylbenzenes, while asymmetrical diphenylacetylenes form a mixture of two isomers.{{cite journal |last1=Xiao |first1=W. |last2=Feng |first2=X. |last3=Ruffieux |first3=P. |last4=Gröning |first4=O. |last5=Müllen |first5=K. |last6=Fasel |first6=R.|date=18 June 2008 |title=Self-Assembly of Chiral Molecular Honeycomb Networks on Au(111) |journal=Journal of the American Chemical Society |volume=130 |issue=28 |pages=8910–8912 |doi=10.1021/ja7106542}}

Image:Diphenylacetylene cyclotrimerization symmetrical.png

Image:Diphenylacetylene cyclotrimerization asymmetrical.png

Image:Hydroformylation Mechanism V.1.svg for the hydroformylation of a terminal alkene ({{chem2|RCH\dCH2}}) to an aldehyde ({{chem2|RCH2CH2CHO}}):{{Ordered list

|Carbon monoxide dissociates from cobalt tetracarbonyl hydride to form the active catalyst, {{nowrap|{{chem2|HCo(CO)3}}}}

|The cobalt centre π bonds to the alkene

|The alkene ligand inserts into the cobalt–hydride bond

|An additional carbonyl ligand coordinates

|A carbonyl ligand migrates into the cobalt–alkyl bond{{cite journal|first1 = Richard F.|last1 = Heck|authorlink1 = Richard F. Heck|first2 = David S.|last2 = Breslow| author-link2=David S. Breslow |title = The Reaction of Cobalt Hydrotetracarbonyl with Olefins|journal = Journal of the American Chemical Society|volume = 83|issue = 19|year = 1961|pages = 4023–4027|doi = 10.1021/ja01480a017}}

|Dihydrogen adds to the acyl complex

|The dihidrydo complex eliminates the aldehyde product,{{cite journal|first = Jack|last = Halpern|authorlink = Jack Halpern (chemist)|title = Organometallic chemistry at the threshold of a new millennium. Retrospect and prospect.|journal = Pure and Applied Chemistry|year = 2001|volume = 73|issue = 2|pages = 209–220|doi = 10.1351/pac200173020209|doi-access = free}} regenerating the catalyst

| An unproductive and reversible side reaction

}}

]]

Hydrogenation of {{chem2|Co2(CO)8}} produces cobalt tetracarbonyl hydride {{chem2|H[Co(CO)4]}}:{{cite book|first1 = M.|last1 = Pfeffer|first2 = M.|last2 = Grellier|chapter = Cobalt Organometallics|title = Comprehensive Organometallic Chemistry III |year = 2007|volume = 7|pages = 1–119|publisher = Elsevier|doi = 10.1016/B0-08-045047-4/00096-0|isbn = 9780080450476}}

:{{chem2|Co2(CO)8 + H2 → 2 H[Co(CO)4]}}

This hydride is a catalyst for hydroformylation – the conversion of alkenes to aldehydes. The catalytic cycle for this hydroformylation is shown in the diagram.

The CO ligands can be replaced with tertiary phosphine ligands to give {{chem2|Co2(CO)8−_{x}(PR3)_{x}|}}. These bulky derivatives are more selective catalysts for hydroformylation reactions. "Hard" Lewis bases, e.g. pyridine, cause disproportionation:

:{{chem2|12 C5H5N + 3 Co2(CO)8 → 2 [Co(C5H5N)6][Co(CO)4]2 + 8 CO}}

File:HCCo3(CO)9.png, {{nowrap|{{chem2|HCCo3(CO)9}}}}, an organocobalt cluster compound structurally related to tetracobalt dodecacarbonyl ]]

Heating causes decarbonylation and formation of tetracobalt dodecacarbonyl:{{cite journal|last = Chini|first = P.|title = The closed metal carbonyl clusters|journal = Inorganica Chimica Acta Reviews|year = 1968|volume = 2|pages = 31–51|doi = 10.1016/0073-8085(68)80013-0}}

:{{chem2|2 Co2(CO)8 → Co4(CO)12 + 4 CO}}

Like many metal carbonyls, dicobalt octacarbonyl abstracts halides from alkyl halides. Upon reaction with bromoform, it converts to methylidynetricobaltnonacarbonyl, {{chem2|HCCo3(CO)9}}, by a reaction that can be idealised as:{{cite journal|first1 = Mara O.|last1 = Nestle|first2 = John E.|last2 = Hallgren|first3 = Dietmar|last3 = Seyferth|first4 = Peter|last4 = Dawson|first5 = Brian H.|last5 = Robinson|title=μ₃-Methylidyne and μ₃-Benzylidyne-Tris(Tricarbonylcobalt) |doi = 10.1002/9780470132517.ch53|journal = Inorg. Synth.| date=2007 |volume = 20|pages = 226–229|isbn = 9780470132517}}

:{{chem2|9 Co2(CO)8 + 4 CHBr3 → 4 HCCo3(CO)9 + 36 CO + 6 CoBr2}}

{{chem2|Co2(CO)8}} a volatile source of cobalt(0), is pyrophoric and releases carbon monoxide upon decomposition.[http://www.coleparmer.com/catalog/Msds/56919.htm Cole Parmer MSDS] The National Institute for Occupational Safety and Health has recommended that workers should not be exposed to concentrations greater than 0.1 mg/m³ over an eight-hour time-weighted average, without the proper respiratory gear.[https://www.cdc.gov/niosh/npg/npgd0147.html CDC - NIOSH Pocket Guide to Chemical Hazards]

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{{carbonyl complexes}}

Category:Cobalt carbonyl complexes

Category:Chemical compounds containing metal–metal bonds