Triple bond complexes Cobalt forms complexes with triple bonded alkynes and cyano compounds. This property is exploited in the use of dicobalt octacarbonyl as protective group for alkynes. In the Nicholas reaction an alkyne group is also protected and at the same time the alpha-carbon position is activated for nucleophilic substitution. [edit]Cyclization reactions Cobalt compounds react with dialkynes and dienes to cyclic intermediates in cyclometalation. Other alkynes, alkens, nitriles or carbon monoxide can then insert themselves into the Co-C bond. Reaction types based on this concept are the Pauson–Khand reaction (CO insertion) and alkyne trimerization (notably with cyclopentadienylcobalt dicarbonyl). [edit]Carbonylations Organocobalt compounds are used as catalysts in carbonylation reactions and more specifically in hydroformylation , the formation of aldehydes from an alkene, formaldehyde and hydrogen. An important catalyst in this reaction type is HCo(CO)4 (cobalthydrocarbonyl) at one time used in the industrial production of butyraldehyde from propylene. In these processes cobalt catalysts are competing with rhodium catalysts such as HRh(CO)(PPh3)4]. In hydrocarboxylations hydrogen is replaced by water or an alcohol and the reaction product is a carboxylic acid or an ester. An example of this reaction type is the conversion of butadiene to adipic acid. Cobalt catalysts (together with iron) are relevant in the Fischer-Tropsch process in which synthesis gas is converted to hydrocarbons. The basic reaction sequence is depicted below [3]: M + CO ? M-CO (M = Co, Fe) M-CO + H2 ? M-CH3 M-CH3 + CO ? OC-M-CH3 OC-M-CH3 ? M-(CO)-CH3 M-(CO)-CH3 + H2 ? M-CH2CH3 [edit]Vitamin B12-type compounds In vitamin B12 cobalt has an octahedral geometry with a Co-C bond in an axial position. In methylcobalamin the ligand is a methyl group. [edit]Sandwich compounds Organocobalt compounds form sandwich compounds. Cobaltocene is a 19-electron metallocene, the compound CoCp(C6(Me)6) ha 20 electrons and 21 electrons are counted in Co(C6(Me)6)2 . The Kläui ligand binds metals. [edit]Cobalt-Mediated Radical Polymerization Main article: Cobalt mediated radical polymerization The weak cobalt(III)- carbon bond is exploited in so- called cobalt-mediated radical polymerization (CMRP) which is a type of controlled radical polymerization.[4] A Co-C bond containing radical initiator breaks up (by heat or by light) in a carbon free radical and a cobalt(II) radical species. The carbon radical starts polymer chain formation with monomer for instance an alkene as in any ordinary radical polymerization. Cobalt is unusual in that it can reversibly reform a covalent bond with the carbon radical terminus of the growing chain. This reduces the concentration of radicals to a minimum and also undesirable termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is called a persistent radical and the cobalt-capped polymer chain is said to dormant. CMRP .

1. Triple bond complexes Cobalt forms complexes with triple bonded alkynes and
cyano compounds. This property is exploited in the use of dicobalt octacarbonyl as protective
group for alkynes. In the Nicholas reaction an alkyne group is also protected and at the same
time the alpha-carbon position is activated for nucleophilic substitution. [edit]Cyclization
reactions Cobalt compounds react with dialkynes and dienes to cyclic intermediates in
cyclometalation. Other alkynes, alkens, nitriles or carbon monoxide can then insert themselves
into the Co-C bond. Reaction types based on this concept are the Pauson–Khand reaction (CO
insertion) and alkyne trimerization (notably with cyclopentadienylcobalt dicarbonyl).
[edit]Carbonylations Organocobalt compounds are used as catalysts in carbonylation reactions
and more specifically in hydroformylation , the formation of aldehydes from an alkene,
formaldehyde and hydrogen. An important catalyst in this reaction type is HCo(CO)4
(cobalthydrocarbonyl) at one time used in the industrial production of butyraldehyde from
propylene. In these processes cobalt catalysts are competing with rhodium catalysts such as
HRh(CO)(PPh3)4]. In hydrocarboxylations hydrogen is replaced by water or an alcohol and the
reaction product is a carboxylic acid or an ester. An example of this reaction type is the
conversion of butadiene to adipic acid. Cobalt catalysts (together with iron) are relevant in the
Fischer-Tropsch process in which synthesis gas is converted to hydrocarbons. The basic reaction
sequence is depicted below [3]: M + CO ? M-CO (M = Co, Fe) M-CO + H2 ? M-CH3 M-CH3 +
CO ? OC-M-CH3 OC-M-CH3 ? M-(CO)-CH3 M-(CO)-CH3 + H2 ? M-CH2CH3 [edit]Vitamin
B12-type compounds In vitamin B12 cobalt has an octahedral geometry with a Co-C bond in an
axial position. In methylcobalamin the ligand is a methyl group. [edit]Sandwich compounds
Organocobalt compounds form sandwich compounds. Cobaltocene is a 19-electron metallocene,
the compound CoCp(C6(Me)6) ha 20 electrons and 21 electrons are counted in Co(C6(Me)6)2 .
The Kläui ligand binds metals. [edit]Cobalt-Mediated Radical Polymerization Main article:
Cobalt mediated radical polymerization The weak cobalt(III)- carbon bond is exploited in so-
called cobalt-mediated radical polymerization (CMRP) which is a type of controlled radical
polymerization.[4] A Co-C bond containing radical initiator breaks up (by heat or by light) in a
carbon free radical and a cobalt(II) radical species. The carbon radical starts polymer chain
formation with monomer for instance an alkene as in any ordinary radical polymerization. Cobalt
is unusual in that it can reversibly reform a covalent bond with the carbon radical terminus of the
growing chain. This reduces the concentration of radicals to a minimum and also undesirable
termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is
called a persistent radical and the cobalt-capped polymer chain is said to dormant. CMRP can be
regarded as a series of carbometalation reactions of vinyl monomers. When the monomer
possesses protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer
may occur. For this reason preferred monomers are acrylic and vinyl esters (e.g. vinyl acetate),

2. acrylic acid and acrylonitrile. The reaction temperature is typically between 0 and 60 °C. The
concept was introduced independently by two groups in 1994.[5][6] Much studied cobalt
compounds are cobaloximes, cobalt porphyrins and Co(acac)2 derivatives, used in combination
with other radical initiators. Cobalt mediated radical polymerization can also follow different
mechanisms, such as catalytic chain transfer or degenerative transfer: [edit]Cyclotrimerisation
Cobalt compounds are catalysts in alkyne trimerisation. In the example of phenylacetylene a
simple catalyst system of cobalt(II) bromide / zinc / zinc iodide suffices to obtain 99% chemical
yield and 97% regioselectivity in favor of the ortho substituted reaction product 1.[7]
Mechanism The reaction mechanism for trimerization is fairly well understood. A 2007
mechanism consistent with in silico and experimental data is depicted below for a cobaltocene
catalyst [8] The ligand L in the 18-electron compound CpCoL2 (A) can be triphenylphosphine
or carbon monoxide. These ligands are replaced by the alkyne in two steps forming first B and
then C which enters the catalytic cycle by oxidative coupling to the 16 VE
cobaltacyclopentadiene metallacycle D. This compound forms in its singlet state and therefore
first relaxes to the triplet state. This intermediate coordinates to another equivalent of ligand to
form E and then forms cobaltanorbornene F (see: norbornadiene) on accepting an alkyne unit in
one [4+2]cycloaddition step. In the final step benzene is liberated in a reductive elimination with
formation of CpCoL which can re-enter the cycle by accepting two units of alkyne. When the
ligand is less of a sigma donor such as ethylene or THF (as a solvent) or when the alkyne is
electron-poor as in butynedioic acid a different cycle takes over where another alkyne unit and
not a ligand adds to intermediate D. In the remainder of this cycle (not depicted) the new
intermediate forms an CpCo(?4arene) complex and then a CpCo(?6arene) sandwich compound
before eliminating benzene and CpCo. Scope In one study, a combination of a [2+2+2]
trimerization and a [4+2] cycloaddition gives access to a Taxol analogue[9] (Scheme 2 [10] ). In
this reaction sequence the trimerization takes place in xylene with catalyst the cobaltocene
CpCo(CO)2 (one Cp unit replaced by two carbonmonoxide ligands) and with irradiation. The
two main components are held together by a temporary silicon tether. Dicobalt octacarbonyl
catalyzes the trimerization towards a hexakis(4-ferrocenylphenyl)benzene
Solution
Triple bond complexes Cobalt forms complexes with triple bonded alkynes and
cyano compounds. This property is exploited in the use of dicobalt octacarbonyl as protective
group for alkynes. In the Nicholas reaction an alkyne group is also protected and at the same
time the alpha-carbon position is activated for nucleophilic substitution. [edit]Cyclization
reactions Cobalt compounds react with dialkynes and dienes to cyclic intermediates in
cyclometalation. Other alkynes, alkens, nitriles or carbon monoxide can then insert themselves

3. into the Co-C bond. Reaction types based on this concept are the Pauson–Khand reaction (CO
insertion) and alkyne trimerization (notably with cyclopentadienylcobalt dicarbonyl).
[edit]Carbonylations Organocobalt compounds are used as catalysts in carbonylation reactions
and more specifically in hydroformylation , the formation of aldehydes from an alkene,
formaldehyde and hydrogen. An important catalyst in this reaction type is HCo(CO)4
(cobalthydrocarbonyl) at one time used in the industrial production of butyraldehyde from
propylene. In these processes cobalt catalysts are competing with rhodium catalysts such as
HRh(CO)(PPh3)4]. In hydrocarboxylations hydrogen is replaced by water or an alcohol and the
reaction product is a carboxylic acid or an ester. An example of this reaction type is the
conversion of butadiene to adipic acid. Cobalt catalysts (together with iron) are relevant in the
Fischer-Tropsch process in which synthesis gas is converted to hydrocarbons. The basic reaction
sequence is depicted below [3]: M + CO ? M-CO (M = Co, Fe) M-CO + H2 ? M-CH3 M-CH3 +
CO ? OC-M-CH3 OC-M-CH3 ? M-(CO)-CH3 M-(CO)-CH3 + H2 ? M-CH2CH3 [edit]Vitamin
B12-type compounds In vitamin B12 cobalt has an octahedral geometry with a Co-C bond in an
axial position. In methylcobalamin the ligand is a methyl group. [edit]Sandwich compounds
Organocobalt compounds form sandwich compounds. Cobaltocene is a 19-electron metallocene,
the compound CoCp(C6(Me)6) ha 20 electrons and 21 electrons are counted in Co(C6(Me)6)2 .
The Kläui ligand binds metals. [edit]Cobalt-Mediated Radical Polymerization Main article:
Cobalt mediated radical polymerization The weak cobalt(III)- carbon bond is exploited in so-
called cobalt-mediated radical polymerization (CMRP) which is a type of controlled radical
polymerization.[4] A Co-C bond containing radical initiator breaks up (by heat or by light) in a
carbon free radical and a cobalt(II) radical species. The carbon radical starts polymer chain
formation with monomer for instance an alkene as in any ordinary radical polymerization. Cobalt
is unusual in that it can reversibly reform a covalent bond with the carbon radical terminus of the
growing chain. This reduces the concentration of radicals to a minimum and also undesirable
termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is
called a persistent radical and the cobalt-capped polymer chain is said to dormant. CMRP can be
regarded as a series of carbometalation reactions of vinyl monomers. When the monomer
possesses protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer
may occur. For this reason preferred monomers are acrylic and vinyl esters (e.g. vinyl acetate),
acrylic acid and acrylonitrile. The reaction temperature is typically between 0 and 60 °C. The
concept was introduced independently by two groups in 1994.[5][6] Much studied cobalt
compounds are cobaloximes, cobalt porphyrins and Co(acac)2 derivatives, used in combination
with other radical initiators. Cobalt mediated radical polymerization can also follow different
mechanisms, such as catalytic chain transfer or degenerative transfer: [edit]Cyclotrimerisation
Cobalt compounds are catalysts in alkyne trimerisation. In the example of phenylacetylene a

4. simple catalyst system of cobalt(II) bromide / zinc / zinc iodide suffices to obtain 99% chemical
yield and 97% regioselectivity in favor of the ortho substituted reaction product 1.[7]
Mechanism The reaction mechanism for trimerization is fairly well understood. A 2007
mechanism consistent with in silico and experimental data is depicted below for a cobaltocene
catalyst [8] The ligand L in the 18-electron compound CpCoL2 (A) can be triphenylphosphine
or carbon monoxide. These ligands are replaced by the alkyne in two steps forming first B and
then C which enters the catalytic cycle by oxidative coupling to the 16 VE
cobaltacyclopentadiene metallacycle D. This compound forms in its singlet state and therefore
first relaxes to the triplet state. This intermediate coordinates to another equivalent of ligand to
form E and then forms cobaltanorbornene F (see: norbornadiene) on accepting an alkyne unit in
one [4+2]cycloaddition step. In the final step benzene is liberated in a reductive elimination with
formation of CpCoL which can re-enter the cycle by accepting two units of alkyne. When the
ligand is less of a sigma donor such as ethylene or THF (as a solvent) or when the alkyne is
electron-poor as in butynedioic acid a different cycle takes over where another alkyne unit and
not a ligand adds to intermediate D. In the remainder of this cycle (not depicted) the new
intermediate forms an CpCo(?4arene) complex and then a CpCo(?6arene) sandwich compound
before eliminating benzene and CpCo. Scope In one study, a combination of a [2+2+2]
trimerization and a [4+2] cycloaddition gives access to a Taxol analogue[9] (Scheme 2 [10] ). In
this reaction sequence the trimerization takes place in xylene with catalyst the cobaltocene
CpCo(CO)2 (one Cp unit replaced by two carbonmonoxide ligands) and with irradiation. The
two main components are held together by a temporary silicon tether. Dicobalt octacarbonyl
catalyzes the trimerization towards a hexakis(4-ferrocenylphenyl)benzene