An improved method for preparing aqueous solutions of diaquocobinamide without hydrolysis products is described. The pKa values have been determined in 0.2 mol dm–3 NaClO4 at 25 °C as pK1= 5.9 ± 0.1 and pK2= 10.3 ± 0.2 and are shown to involve one proton each. Evidence is presented that diaquo-, aquohydroxo-, and dihydroxo-cobinamide exist in solution as isomers depending on hydrogen bonding between the axial ligands and different amide side-chains.
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Baldwin et al. (1983) studied this question.