Cobalt bond dissociation energies

Cobalt-Carbon Bond Homolysis Studies with coenzyme Bi2 model compounds and TEMPO as a radical trap have allowed the determination of rate constants and activation parameters, which in turn led to estimates of the Co-C bond dissociation energies (BDEs). The key role of the homolytic Co-C cleaving step in these reactions was established by the following observations (Co11) is produced addition of external... 

Equilibrium. Reactions of the type depicted by Equation 16 have been identified as synthetic routes to or-ganocobalt compounds (17, 18). Cobalt-alkyl bond-dissociation energies could be deduced from the enthalpies of such reactions if the latter could be determined, for example from the temperature dependence of the corresponding equilibrium constants. 

According to this interpretation, AH 24 may be identified with the cobalt-alkyl bond-dissociation energies in these compounds. However, at this stage other mechanistic interpretations, such as that involving a concerted olefin elimination, cannot be excluded (29). The investigation of these systems is continuing. 

Finally, in the context of these results, the well recognized, apparently lower thermal (and photochemical) stability of secondary cobalt-alkyls, relative to primary ones, may reflect the greater accessibility of irreversible decomposition pathways involving olefin elimination (i.e., through schemes such as that in Reactions 25-26), in addition to some probable lowering of the metal-alkyl bond-dissociation energy. 

Ng, F.T.T., Rempel, G.L., and Halpern, J., 1983, Steric influences on cobalt-alkyl bond-dissociation energies, Inorganica Chimica Acta-Letters 77 L165riL166. 

In view of the coordination pattern in protein-bound methylcobalamin (see below), the thermodynamic studies on the effect of the coordination of the dimethylbenzimidazole base to the a-side of the cobalt center on the homolytic and heterolytic (Co/3-C)-bond dissociation energy in (4) (thermodynamic effect of the trans hgand or trans influence ) were extended to corresponding investigations with CojS-methyl-imidazolylcobamides, such as (9), where imidazole replaces the dimethylbenzimidazole these studies showed this change of the nature of the axial base to have httle effect on the two relevant bond-dissociation energies of the corresponding methylcobamide. ... 

Bridged species such as 76 are well documented in rhodium porphyrin chemistry.240-241 An acetylene bonded to one metal-centered radical is presumed to be trapped by addition of a second metal-centered radical. Lower bond dissociation energies of cobalt relative to rhodium would disfavor species such as 76 and facilitate the reaction with metal—hydride intermediates to form a trans product. 

The thermal decomposition of cobalt carbonate fitted the Avrami-Erofeev equation with n = 1.37 at 493 K and 2.54 at 543 K [74]. These data were later reanalyzed by Engberg [75] who allowed for an initial reaction, 0.02 and found , = 95 kJ mol, which is close to the Co-O bond dissociation energy (91 kJ mol ). 

Determination of Cobalt-Carbon Bond Dissociation Energies... 

It is only during the past ten years that reliable and widely applicable methods for determining homolytic metal-alkyl bond dissociation energies of stable or-ganometallic compounds in solution have been developed and that information about such bond dissociation energies has become available. Today about one hundred transition metal-alkyl bond dissociation energies have been determined, the majority for cobalt-alkyl complexes. Most of these have been from kinetic measurements. The scope, limitations and results of such determinations are discussed. 

Table IV. Comparisons of Bond Dissociation Energies of Alkyl-Cobalt Complexes Determined by Different Methods...

COBALT CARBON BOND ENERGIES. The cobalt-carbon bond dissociation energy has been reported (18) for a series of base adducts of alkyl-substituted bis (dimethylglyoximato) cobalt(II),... 

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