In acidic aqueous solution, a cobalt(III) complex containing monodentate N(9)-bound adeninate (ade(-)), cis-[Co(ade-kappaN(9))Cl(en)(2)]Cl (cis-[1]Cl), underwent protonation to the adeninate moiety without geometrical isomerization or decomposition of the Co-III coordination sphere, and complexes of cis-[CoCl(H(2)ade)(en)(2)]Cl-2 (cis-[2]Cl-2) and cis-[Co(H(2)ade)Cl(en)(2)]Cl-3 (cis-[3]Cl-3) could be isolated. The pK(a) values of the Hade and H(2)ade(+) complexes are 6.03(1) and 2.53(12), respectively, at 20 degreesC in 0.1 M aqueous NaCl. The single-crystal X-ray analyses of cis-[2]Cl(2)(.)0.5H(2)O and cis-[3]Cl-2(BF4)(H2O)-H-. revealed that protonation took place first at the adeninate N(7) and then at the N(1) atoms to form adenine tautomer (7H-Hade-kappaN(9)) and cationic adeninium (1H,7H-H(2)ade(+)-kappaN(9)) complexes, respectively. On the other hand, addition of NaOH to an aqueous solution of cis-[1]Cl afforded a mixture of geometrical isomers of the hydroxo-adeninato complex, cis- and trans-[Co(ade-kappaN(9))(OH)(en)(2)](+). The trans-isomer of chloro-adeninato complex trans-[Co(ade-kappaN(9))Cl(en)(2)]BF4 (trans-[1]BF4) was synthesized by a reaction of cis-[2](BF4)(2) and sodium methoxide in methanol. This isomer in acidic aqueous solution was also stable toward isomerization, affording the corresponding adenine tautomer and adeninium complexes (pK(a) = 5.21(1) and 2.48(9), respectively, at 20 degreesC in 0.1 M aqueous NaCl). The protonated product of trans-[Co(7H-Hade-kappaN(9))Cl(en)(2)](BF4)(2)(H2O)-H-. (trans-[2] (BF4)(2)(H2O)-H-.) could also be characterized by X-ray analysis. Furthermore, the hydrogen-bonding interactions of the adeninate/ adenine tautomer complexes cis-[1]BF4, cis-[2](BF4)(2), and trans-[2](BF4)(2) with 1-cyclohexyluracil in acetonitrile-d(3) were investigated by H-1 NMR spectroscopy. The crystal structure of trans-[Co(ade)(H2O)(en)(2)]HPO(4)(.)3H(2)O, which was obtained by a reaction of trans-[Co(ade)(OH)(en)(2)]BF4 and NaH2PO4, was also determined.