Reactions of the cobalt (II)-bipyridine system in basic aqueous solution

Abstract

Basic solutions of Co(bipy)2++, where bipy is 2,2'-bipyridine, react with molecular oxygen, undergoing color changes from the original yellow; rapidly to dark brown, and finally much more slowly to red~ Well formed red crystals may be separated from the final solution~ The initial reaction involves the reaction of one molecule of hydroxide ion and one molecule of oxygen per two atoms of cobalt. The pH decrease of the yellow to brown step serves as the basis for monitoring the reaction. Utilizing the pH-stat technique for monitoring the reaction, the fast step of the reaction is indirectly followed as volume of base added versus time. The reaction is found to be first order in both hydroxide ion and oxygen concentration and second order in complex concentration. Magnetic susceptibility measurements showed that the original yellow complex had three unpaired electrons, while the final red complex (both in the solid state and in solution) is diamagnetic. The indications are that the brown intermediate is less paramagnetic than the original yellow complex. The activation energy of the yellow to brown step of the reaction is approximately 11 Kcal mole^-l.