The isomerisation of the a and b forms of several acid cyano-aquo-corrinoids (with one to four carboxyl groups or one phosphoric acid group) in aqueous medium was investigated. The reaction is accelerated by light, heat, anions (e.g. acetate ion) and carbon monoxide. For all cyano-aquo-corrinoids with carboxyl groups investigated the equilibrium a ⇄ b shifts in the direction a → b when the carboxyls dissociate; in the case of carbonic acids prepared from cobyric acid, the shift grows with the number of the carboxyls. On the contrary, the cyano-aquo-corrinoids with a phosphoric acid group (cobinamide phosphate and its three isomers) exist as a mixture of about 55 per cent a form and about 45 per cent b form in the entire pH range (2.5 to 9) investigated. It is shown that the isomerisation of the cyano-aquo-corrinoids follows the known rules of ligand exchange (and of the connected reactions, such as steric change, racemisation, and disproportionation) for cobalt (III) complexes.
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