In this talk, we will discuss the relationship between structure and CoC bond cleavage in organocobalt compounds. At least two organocobalt compounds are required in humans [1]. These are deoxyadenosylcobalamin (coenzyme B 12) and methylcobalamin (methyl B 12). Both compounds have CoC bonds in one coordination position. The ligand is simply a methyl group in methyl B 12 but is 5′-deoxyadenosine in coenzyme B 12. The deoxyadenosine is bound to Co via the 5′ carbon of the ribose sugar. It is generally accepted that the cobalt in these compounds is in oxidation state III (low spin d 6). The four coordination positions cis to the carbon donor ligand are occupied by four N's of a corrin ligand. The position cis to these four N donors and trans to the alkyl ligand is occupied by N(3) of a 5,6-dimethylbenzimidazole which is linked to the corrin ring via a complex chain containing a phosphodiester linkage.Thus the macrocyclic corrin and its pendant benzimidazole chain is a pentadentate ligand. There is considerable flexibility in the corrin ring system and the structures of metallocorrin compounds have recently been reviewed in depth [2]. Coenzyme B 12 is known [1] to be required in about a dozen enzymatic reactions, mostly in bacterial systems. Although there are substantial differences between the enzymes both in the nature of the reactions catalyzed and in the requirement for additional cofactors [1], there is little doubt that, for several catalytic processes, homolytic cleavage of the CoC bond is an essential step. The initial products of this cleavage are cob(II)alamin B 12r and the deoxyadenosine radical. Since the coenzyme is reasonably stable to CoC bond cleavage under physiological conditions (in the absence of light), most workers in the field believe that interaction between the coenzyme and the enzyme promotes or ‘triggers’ the cleavage reaction [3, 4]. The most convincing explanation for the enzyme-promoted cleavage involves a conformational change of the coenzyme induced in the holoenzyme. Such a conformational change could weaken the CoC bond either through steric or electronic effects, or a combination of the two. The extensive evidence that the strength or stability of the cobaltcarbon bond is sensitive to steric or electronic changes in the ligands, including the alkyl group, attached to the cobalt cannot be reviewed here. The reader is referred to an extensive literature [3–5]. In particular, using elegant kinetic and equilibrium studies of model organocobalt compounds, Halpern has assessed the influence of both the electronic and steric properties of trans ligands on CoC bond strengths [6–8]. These studies are described in another talk in this symposium, which emphasizes the dynamic and energetic aspects of CoC bond cleavage. In this talk, we will focus on structure. At the time of Glusker's review of structure [2], only one organocobaltcorrin had been structurally characterized by X-ray crystallography. However, this compound is the very important coenzyme B 12 itself [9, 10]. The unexpected presence of the CoC bond was revealed in this study - however, no organocobaltcorrin structures have been reported since. We have crystallized and are structurally characterizing methyl B 12, the other recognized coenzyme in human metabolism. Structural details will be available at the time of the meeting. Acknowledgments. This research was supported by NIH grants GM 29225 (LGM) and CA 10925 (JPG) and by the CNR, Rome (LR), and by NATO (LGM and LR). We are grateful to these organizations for this support.