THE CHEMISTRY OF Co(I) DERIVATIVES OF VITAMIN B12 AND OF RELATED CHELATES

Publisher Website
Google Scholar
Abstract
Summary: The main points of the present review can be summarized as follows:1. The existence of Co(I) derivatives is limited to strong‐field chelates, since the stabilization of spin‐paired d8‐ions is particularly favored in planar complexes. The Co(I) chelates do not form stable adducts with Lewis bases, but weak interactions yielding five‐coordinate species are observed.2. There appear to be no basic differences between the reactions of the Co(I) center in vitamin B12s and the corresponding derivatives of simple chelates. Thus, the rates of the reactions of primary alkylating agents with vitamin B12sor cobaloximes(I) are very nearly the same.3. The dz2 orbital in the Co(I) compounds is weakly antibonding, extending its charge cloud perpendicular to the plane of the molecule. This causes the Co(I) chelates to be the strongest nucleophiles known.4. All Co(I) chelates studied show three or four absorption bands between 12000 and 18000 cm‐1 that are assigned to the ligand‐modified d‐d transitions. In the isolectronic nickel chelates, corresponding transitions were observed between 20000 and 28000 cm‐1.5. The reduction of vitamin B12 and of cobaloximes can be achieved with a variety of reagents, e.g., NaBH4, zinc dust, thiols. New reducing agents are acyloins or ketosugars. The reduction with NaBH4 is sensitive to trace‐metal catalysis. Cobaloximes(II) (but not vitamin B12) are reduced by molecular hydrogen, a fact presumably associated with the existence of dimeric, metal‐metal bonded Co(II) derivatives.6. The reduction of Co‐Salen and Co‐BAE to the Co(I) derivatives in water requires strongly alkaline conditions to prevent the decomposition into the Co(II) derivatives and hydrogen. This is a direct consequence of the higher Co(II)/Co(I) reduction potentials of these compounds.