The Hydration of Carbon Dioxide-Catalysis by Transition Metal Hydroxo Complex Ions

Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214

Department of Chemistry, University of the West Indies, Kingston 7, Jamaica

The equilibration rate between dissolved carbon dioxide and ionic carbonate in aqueous systems is far from instantaneous in the absence of effective catalysts. In natural plant and animal systems, the catalysis is provided by the enzyme carbonic anhydrase. On this, the active site is known to be the ZnN₃ (OH) moiety, where the N's represent histidyl nitrogen ligands from units within the polypeptide chain of the enzyme. Some simple transition metal hydroxo comptex ions also serve to convert CO₂ to ionic carbonate, though none so far reported shows activity anything like as great as that of the metalloenzyme. With cobalt(lll) complexes, the product is always a rather stable carbonato complex. We have now completed studies of kinetics of such "carbon dioxide uptake" reactions for a series or complexes, comprising the types Co(N₅) (OH) ²⁺, Co(N₄) (OH₂) (OH)²⁺ and Co(N₄) \((OH)^+_2\) waere N₄ ani N₅ represant amine ligand grouping of various kinds. The data obtained are discussed with respect to their mechanistic significance and some general conclusions are drawn concerning the nucleophilicity or hydroxo metal complexes toward carbon dioxide in aqueous systems.