Virtually all the biological processes in which

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Unformatted text preview: les”), liberating Hz from water. Virtually all the biological processes, in which vitamin B12 is active, involve substituent exchange of the type: Figure 26.7 Model vitamin B I2 compounds: (a) a Schiff base derivative, and (b) a cobaloxime, in this case derived from dimethylglyoxime. cobalt giving it three accessible and consecutive oxidation states: which, significantly, does not involve solvent protons. The precise mechanism of these reactions is not settled but all involve cleavage of the Co-C bond and it is evident from the study of model systems that the lack of complete planarity of the corrin ring is an important factor in controlling this.(36) 2 . . Organometallic compounds(37) 635 Many of the organometallic compounds of the elements of this group show valuable catalytic activity and, as discussed above, much of the chemistry of vitamin B12 is the chemistry of the Co-Ca bond. Simple homoleptic alkyls and aryls of cobalt, [COR,], have not in fact been prepared, but this is evidently not due to thermodynamic instability of the Co-C bond. Compounds containing such bonds can be prepared in abundance, not only with (a n)bonding ligands such as phosphines and CO but also with non-n-bonding ligands such as Schiff bases and glyoximes. These latter presumably owe their existence not to electronic but rather to steric factors, the additional ligands blocking what might otherwise be energetically favourable decomposition paths. + The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Cor’ with Schiff bases, Or substituted glyoximes (giving coba...
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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