1,1-Dimethylpropyl hydroperoxide reacts with cobalt(II) cobinamide via a Fenton-like reaction to produce a mixture of the diastereomeric alpha- and beta-ethylcobinamides (CH(3)CH(2)Cbi(+)’s, in which the organic ligand is in the "lower" or "upper" axial ligand position, respectively), the composition of which depends on the ratio of starting materials. When the hydroperoxide reagent is in excess, <2% of the CH(3)CH(2)Cbi(+) product is the a diastereomer, a product distribution which agrees with previous observations of the equilibrium mixture. This distribution apparently arises because the previously demonstrated CH3CH2.-promoted isomerization of CH(3)CH(2)Cbi(+) diastereomers leads to equilibration. However, when cob(Il)inamide is in excess over hydroperoxide, the CH(3)CH(2)Cbi(+) product contains 87% alpha diastereomer and 13% beta diastereomer. Evidently, under these conditions, trapping of the CH3CH2. radical is sufficiently rapid to prevent the radical-induced isomerization of diastereomers and a kinetically controlled product distribution results. This condition has been used to study the relative energetics of kinetically controlled alpha and beta alkylation of cob(II)inamide by CH3CH2.. For cob(II)inamide itself, the a diastereomer is enthalpically stabilized relative to the beta diastereomer in the transition state for carbon-cobalt bond formation, but an even larger entropic stabilization of the beta diastereomer causes the latter to be the predominant product. The entropic preference for the beta diastereomer is shown to be the result of the differential side chain configurations at the alpha and beta faces of the cobalt corrinoids by experiments with side-chain-altered analogs. When the downwardly projecting f side chain is enlarged by esterification of the N-methylated nucleotide 1-alpha-D-ribofuranosyl-3,5,6-trimethylbenzimidazole 3’-phosphate, the proportion of the a diastereomer in the alkylated product drops to 74% and the effect is due solely to an increased entropic preference for the beta diastereomer. Similarly, when the normally downwardly projecting e propionamide side chain is epimerized to the upward (beta) face of the corrin, the proportion of the alpha diastereomer in the product is increased to 95% and the effect is entirely entropic again. Taken together with previous work, the results lead to a general picture of the energetics of alkyl radical f cobalt(Il) corrinoid combination reactions and their microscopic reverse, the homolytic dissociation of carbon-cobalt bonds, a rare instance in which the subtleties of such diastereomeric control can be understood at a very fundamental level.