The Cobalt Triad

The reaction of state-selected Co ions with Mel in the gas phase gives an adduct as well as CoMe and C0I+ species the bond energy in CoMe was calculated as 223 8 kJmol 1.175 A theoretical study of CoCH2 and its reaction with hydrogen has appeared. 176 Hiotolysis of HCoMe, formed by cocondensing Co atoms, CH2N2 and H2, has enabled the detection of a o-complex intermediate in the reaction of Co atoms with CH4.177 

A density function theoretical study of [Co(CO)4Me] and related compounds has been performed a value of 198 kJmol for the Co-Me bond dissociation energy was derived.224 The heterogeneous reaction of Na[Co(CO)4] with 4-bromo-2-pentene gives mainly the insertion product 

Addition of PhN3 to [CpCo(CNCH2Ph)(PMe3)] gave the side-bound carbodiimide lecies [CpCo(PhNC=NCH2Ph)(PMe3)], while the related thione complex 

An ab initio study of the cobaltacyclopentadiene [CpCo(C4H4)] and its bis-alkyne isomer [CpCo(Ti-C2H2)2] has concluded that the former is more stable by around 80 kJmol. 239 A novel polymer, (18), has been prepared by treating (CpCo(CO)(PHi3)] with 4,4 -diethynylbiphenyL2 Formation of a cobaltacyclopentadiene from a 2,10-diethylnyI-l, 6-methanoaimulene and its insertion of xylylNC has also been reported.24  

The chemistry of [LRhMe2(OTf)] and [LRhMe(0 n)2] (L = 1,4,7-trimethyI-1,4,7-triazacyclononane) in MeOH and H2O has been explored the latter forms a slow one-component catalyst for ethylene polymerisation in these solvents. The species in solution is [LRh(H20)2Me]2+ which on removal of solvent gives [LRh(OH)Me(OTf)].25l The reaction of [Rh(bipy)(C2H4)(Cl)] with CO2 forms the rhodalactone [ bipy)Rh(CH2CH2COO)].252 

The reactions of state-selected Co ions with propane in the gas phase have been studied C-C activation only occurs with excited state ions whereas C-H activation also occurs with ground state, Reactions of Co with primary amines give ammonia, probably by insertion into the C-N bond followed by P-elimination.i98 

A novel B 2 model system consisting of a tetraazamacrocycle linked to an axial alkyl group has been synthesized. A Co(II) macrocycle complex reacted with methyl radicals under CO to give a mixture of Co-Me and Co-COMe complexes.238 Alkyl radicals generated from [RCo(L)(H20)]2+ 

Theoretical studies on the oxidative addition of methane to second row metal atoms show that Rh has the lowest activation energy the potential surface for Rh atoms is remarkably similar to that of RhCl(PR3)2-based C-H activating complexes.268 The dissociative chemisorption of acrolein and allyl alcohol on a Rh(l 11) surface has been investigated.  

Mechanistic studies suggest that the reaction of [(HBpz 3)RhH(Ph)(CNR)] (R = CH2BUO with RNC, giving [(HBpz 3)Rh(CNR)2], proceeds via an Ti -benzcne intermediate.299 a 1,3-dipolar addition of PhN3 to [(HBpz 3 Rh(CNR)2] gives [(HBpz 3)Rh(CNR)(Ti2.phN=C=NR)] which is capable of photochemical C-H activation of and reacts with more RNC to form the metallacycle [(HBpz 3)Rii(CNR) C(=NR)NPhC( NR))].300 

CH2=CHCX)2Me)], which on heating undergoes C-C bond formation to give (13), thus shedding light on the mechanism of acrylate dimerisation.315 

The reactions of Co ions in the gas phase with propane and with cyclohexane have been examined by a number of mass spwctrometry techniques.223.224 

Numerous papers deal with the chemistry of vitamin B]2 cobalamins, cobinamides, and their synthetic models, the cobaloximes [Co(dmgH)2(R)(L)], including the synthesis of new species,225-228 structural studies,229-239 the stereochemistry of formation and isomerisation,240-244 and reactivity.245-248 jhe effects of viscosity,249 pressure, temperature,250,25l and base coordination252 

Theoretical studies have estimated an activation energy of about 88 kJmol for the migratory insertion reaction of [CoMe(CO)4] to give [Co(CO)3(COMe)].92 The use of [Co(CO)4(CF3)] in MOCVD of cobalt silicide has been examined.292 The reaction of [Co(CO)4(Tii-C3H4-2-R)l (R H, Me) with dppm or dppe results in coordination of Co to one or both ends of the phosphine.293 

An oxametallacyclic intermediate has been suggested for the oxidation of alkenes to ketones on an oxygenated Rh(l 11) surface.3l3.3l4 Theoretical calculations on the oxidative addition of CH4 to second row metal atoms predict that this will be most favourable for Rh.3l5 The exothermicity of the addition of CH4 to Rh(I) can be improved by the presence of a ligand, but its position (in or out of the RhCH plane) is crucial.316 

A number of cyclometallated Rh complexes have been prepared with ligands such as 2-phenylpyridine,361 2-benzylpyridine,362 l-phenylpyrazole,363 6-(2-thienyl) bipy, and 2,6-diphenylthiophenol.364 The photophysical properties of some phenylpyridinate complexes have been studied.365 The hydrometallation of vinylcyclopropane with a cyclometallated Rh complex gave quinolyl pentenyl ketones,366 and the insertion of CO into the Rh-C bond of a cyclometallated phosphine has been reported.367 

The Cobalt Triad.— The first example of a paramagnetic octahedral cluster, [COeC(CO)i4], shows uneven lengthening of cobalt-cobalt bonds indicative of the anti-bonding character of the extra electron. Transition-metal carbonyls, particularly ICo2(CO)8l catalyse the conversion of CO and H2 at moderate temperatures and pressures into ethanol and other oxygenated products. Parameters obtained by a complete Wolfsberg-Helmholtz (Extended Hiickel) MO analysis of [CoMe(CO)4] have been fitted to the measured u.v. absorption bands. Products from the co-condensation of cobalt vapour with cyclo-pentadiene and various acetylenes include (8—11).  

Hammer, J. Gross, and U. Schubert, Angew. Chem., Int. Ed. Engl., 1980, 

Organoiridium(i) diene complexes [IrR(diene)2] have been prepjffed by treatment of the corresponding chlorides [IrQ(diene)2] or a mixture of [ IrCl(cod) 2l and diene with lithium-methyl or -phenyl. Oxidative addition of alkyl halides (except MeX), vinyl halides, aryl halides, and a-halo-esters to [IrCl(CO)(PR3)2] complexes shows characteristics consistent with a radical 

2 The Cobalt Triad. - Oxidative addition reactions between hydrocarbon radicals and complexed cobalt(II) centres in the gas phase have been studied using negative chemical ionisation mass spectrometry. The gas-phase chemistry of Co with a series of 1-chloro-n-alkanes and alcohols has been investigated, and it has been found that for RCl, as the chain length increases, Co insertion into internal C-C bonds becomes preferred to insertion 

The cis-dialkylcobalt (III) complexes, cis-CCoR (bipy 2 ] (C10 ) (R=Me,Et, and Bz) undergo photo-induced Co-C cleavage which induces 

3-trifluoroprop-l-yne to give [Co (C2CF3)(salen) (py) ],and [ co(salen)J 2 2 [Co(salen) (acac) ] react with PhCSCH to give 

2SiMe3) have been reported. The complex [RhCl ( 7 -082)-(triphos)] underoges a reaction with F3CCSCCF3 to give the rhodacyclobutene complex (11).  

Hydride abstraction using [CPh3]BF occurs from the -carbon 

2 The Cobalt Triad. - Numerous studies concerning the synthesis and reactivity of Group 9 complexes containing metal-alkyl bonds have appeared and are summarised below. 

The role of the cyclopentadienyl ligand in the a-bond activation of methane by [Co(ri -C5H5)] has been studied and a reaction mechanism proposed which includes several intermediates containing Co-alkyl bonds.A series of Ir-Me species have been synthesised as part of an investigation into the effect of the hydridotris(pyrazolyl)borate ligand on C-H bond activation by cationic iridium complexes. The rhodium species 23 has been synthesised as part of a study into 

R = primary, secondary or tertiary alkyl group B = Base ligand (pyridine or water) 

Several articles dealing with the topic of metal-aryl bonds have appeared and are collected together below. 

The unprecedented formation of five-, six- and seven-membered metallacyclic complexes via single and double insertion of mono- and disubstituted alkynes into a Rh-O bond has been reported. The mono- and dinuclear dichalcogenide complexes [Ir r -EC(R) = C(R )E (r -C5Me5)], [Ir ri - 

1- methylheptylbis(dimethylgloyximato)cobalt(iv) by chloride anions to form 

Ohtani, A. Yamagishi, and M. Fujimoto, Bull. Chem. Soc. Japan, 1978, 51, 2562. 

The novel rectangular cluster (32) has been obtained by treatment of [Ir4(CO)ia] with (MeOaCC CCOaMe). Formation of the same n-alkyliridium(m) complexes [IrCaaR(CO)(PPh8)a] by oxidative addition of normal or a-branched acyl 

Ogoshi, J-I Setsune, Y. Nanbo, andZ.-I. Yoshida,/. Organometallic Chem., 1978,159,329. H. J. Callot and E. Schaeffer, J.C.S. Chem. Comm, 1978, 937, 

Neopentyl-13-epi- and neopentyl-8-epicobalamin have been synthesised and the mechanism and kinetics of their thermolysis studied. The enthalpy change on homolysis is unaffected by the epimerisation but the entropy change is affected. The work has been extended to look at the base on/base off equilibria of alkyl-13-epi-and alkyl-8-epicobalamins. There is a seccmdaty p-deuterium isotope effect on the nmr resonance of nitrogen ligands axially coordinated to ethylcobalamin this provides evidence for a classical hyperconjugation contribution to the cobalt -alkyl bond.  

The structures of twelve compounds of the type [Co(R)(L)(Hdmg)2] have been optimised as part of a study of the electronic structure of the co-enzyme of vitamin B12 and related doivatives.  

Photolysis of [Co(CH2R)(L)(Hdmg)2] under oxygen proceeds by insertion of dioxygen into the cobalt carbon bond to provide a solution species for which nmr spectroscopic data is rq)orted. Reduction of this intermediate produces primary alcohols whereas thermolysis produces aldehydes and alcohols. Treatment of [Co oep)] with simple aldehydes and rm-butylhydroperoxide in the presence of sodium borohydride produces cobalt(III) acyls in 65-98% yields. In the absence of the borohydride the yield is reduced. The reaction is proposed to proceed by acyl radical trapping by the Co(n) centre. Methyl transfer in a protein free model of vitamin B12 dependent methyl transf enzymes has been studied. These systems convert homocysteine to methionine in nature. Trimethyl-phenylammonium icm reacts with the CoG) centre in cobalamin producing methylcobalamin.  

Catalytic amounts of cobalt halides react with methyl lithium and tominal aliphatic epoxides to produce methyl ketones and alcohols. The thermochemistry and structures of the metallocycle and metal alkene isomers of formula Co(C3H6) from reaction of gaseous Co and propene or cyclopropane in a flow system have been studied.  

Trans- oxidative addition of CH2I2 to [Rha(CO)(PR3)2] occurs under electronic control to produce [RhCl(CH2l)(I)(CO)(PR3)2] when R3 = Me3, Et3, Et2Ph, but does not occur for R3 = Ph3, EtPh2, Cy3, or (OMe)3. Carbonylation at 100°C and 70 atmospheres pressure produced acyl intermediates vtrhidi lost ketene Mediane adds oxidatively across the metal centre in 

In an ion beam apparatus, Co ions react with acetone to form 

and dppe) have been investigated. The cationic complex [Co(CgMeg)Et P(p-tolyl)2 is formed on protonation of [Co(CgMeg)-(C2H ) P( -tolyl)2)]In this cationic ethylcobalt complex there is a strong 3-centre interaction between the metal and a 8-C-H, as indicated by and C n.m.r. and a single crystal X-ray study.  

Treatment of [CoBr(Cp) (r -allyl) ] with RLi or RMgX leads to the 

Novel dinuclear complexes [(n-CgH.R)Co(PPh26gH )(M-CMe)Co(n-CgH R)], are also formed in this reaction. [Mg(C10 )2] acts as a catalyst 

A structural characterisation of the cobalt(III) Bj 2 niodel compound, trans-adamantyIbis(dimethyIglyoximato)(1-methylimidazole) cobalt(III), has shown that it has a very long Co-C bond of 

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In mixed metal clusters, redistributions can occur in the mass spectrometer so that the highest observed peak arises from such a product. This is clearly exemplified by the heterometallic dodecacarbonyls containing metals of the cobalt triad. The thermal stability of this series of clusters decreases as the rhodium content increases and facile redistributions result in the formation of tetranuclear species containing less rhodium the very thermally stable Q I CO) does not rearrange184. ... 

Kinetic studies of the substitution reaction of (CO)3M[ 3-C3(t-Bu)3] (M = Co, Rh, Ir) with P(OEt)3 provide the first examples of dissociative CO substitution for carbonyl complexes of the cobalt triad (equation 256)323. 

Chini et at. synthesized a series of mixed-metal clusters that contain various combinations of the metals within the cobalt triad (113). The... 

Oxidation via intermediate formation of hydrido species has been achieved but only with clusters of the cobalt triad. At least two isomers of IrgfCO) (235) have been isolated from the oxidation of [Ir6(CO)i5] by varying the conditions employed, [Eqs. (22) (235) and (23) (236) ]. 

The [M(NH3)5(0S02CFa)] + ions actually show an increase in relative lability, compared with [M(NH3)5C1]2+ ions, for second- and third-row members of the cobalt triad the ratio of aquation rate constants (CF3S03")/ (C1 ) varies from 1.4 x 10 (Co) to 5.0 x 10 (Rh) and 2.1 X 10 (Ir) (84). Likewise, the ratio of aquation rate constants for triflate and nitrate is substantial, with (CF3S03 )/ (N03 ) varying... 

Comparison of data in Tables II and III for the cobalt triad indicates that aquation proceeds, on average, about the same for Rh(III) as for Co(III) and substantially slower for iridium(III). As has been observed earlier, however, lability of CF3SO3" complexes is actually enhanced relative to other complexes (such as Cl" complexes) for iridium(III), and for second- and third-row metal ions generally. The property of CF3SO3" as a good leaving group is essentially constant, but the rate of displacement is clearly mediated by the metal ion aquation of the iridium(III) complex is over 100 times slower than for cobalt(III) whereas loss from the exceptionally inert platinum(IV) is slower still (84). 

One notable area of chemistry that can offer some prospects for enhanced displacement of ligands is photochemistry. The photochemistry of metal complexes has been extensively studied and reviewed (16, 282), and it is not proposed to cover this area in any detail. One of the complications in photochemistry of metal complexes is the marked change in reactivity even within a triad of the d block. For example, the reaction modes of the simple [M(NH3)5N3]2+ ions for the cobalt triad vary clearly. For cobalt(III), the photochemistry is dominated by an intramolecular redox reaction (93) whereas for iridium(III), reac-... 

The elements of the cobalt triad are most inclined to assemble stable five-membered rings according to Eq. e.g., the two-step sequence in Eq. (d) " ... 

The interest1 in transition-metal metallacycles has led to an intensive study of a number of such complexes from the structural and chemical point of view. In this context, the availability of metallacyclopentane derivatives of the cobalt-triad metals, which are implicated in so many catalytic and stoichiometric reactions, enables one to extend the range of studies of the chemistry of this class of organometallics. 

The redox properties of complexes of the cobalt triade with [2 ]cyclophanes [78] as well as iron [180] were also examined and show a similar behaviour as... 

Cluster Compounds.—The reported preparation of the first carbidocarbonyl derivative of the cobalt triad, namely [Rh6(CO)i5C] -, involves the action of... 

Papers dealing with the general reactions of phosphorus group ligan s in the realm of the metal carbonyls are also represented in this report. Basolo and co-workers have examined the kinetics of CO substitution in a number of iridium carbonyl complexes and have found the first examples of a dissociative mechanism for CO substitution in the cobalt triad carbonyl complexes. Bridged diarsinidene complexes are also reported. ... 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

The Cobalt Triad. - Treatment of the metal chlorides C0CI2 or RhCl2.3H20 with CNCH2CHROH (R=H or Me) yields the homoleptic carbene complexes [m cN(H) CH2CHRO g]Clg (M=Co,Rh).The mononuclear carbene complex [Co(Cp)(CO)(=CN(Me)CH2CH2NMe]3 has... 

The Cobalt Triad. - The complex [Co(1-norbornyl) )] has been shown to be a low-spin tetrahedral complex by a combination of X.-ray crystallography and magnetic susceptibility measurements. 

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