The chemistry of vitamin B12. Part 24. Evidence for hydride complexes of cobalt(III) corrinoids

Abstract

The u.v.–visible absorption spectra of solutions produced by the reduction of aquocobalamin (B_12a) or cyanoaquocobinamide (Factor B) in glacial acetic acid with zinc dust show the presence of an unstable yellow complex, which has a spectrum similar to those of alkylcobinamides and is therefore identified as a five-co-ordinate Co^III corrinoid with hydride as the axial ligand. The reported ability of such solutions to react with unactivated olefins such as ethylene and propylene has been confirmed by the isolation and identification of ethylcobalamin and isopropylcobinamide as products. The equilibrium constant for the co-ordination of the hydride ion in aqueous solution is calculated to be log₁₀(K/dm³ mol^–1)∼ 41 at 25 °C, where K=[Co–H]/[H₂O–Co–OH₂][H^–](axial ligands only given).