Three cobalt(III) complexes of themacrocyclic tetraamine [3(5)]adamanzane (1,5,9,13-tetraazabicyclo[7.7.3]-nonadecane) were isolated as salts. The X-ray crystal structures were solved for the compounds [Co([3(5)]adz)-(CO3)AsF6 (1b), [Co([3(5)]adz)(HCO3)]znBr(4).H2O (2a), and [Co([35]adz)(SO4)]AsF6.H2O (3a). The coordination geometry around the cobalt(III) ion is a distorted octahedron with the inorganic ligands at cis-positions. Complex 2 is the second example of a cobalt(III) complex for which the X-ray structure,sfiows a chelate binding mode of the hydrogen carbonate entity. The pK(a) value of the [Co([3(5)]adz)(HCO3)](2+) ion (2) was determined spectrophotometrically to be 0.27 (25 degreesC, I = 5.0 M). The protonation appears to occur at the noncoordinated carbonyl oxygen atom of the carbonate group, with hydrogen bonding to the crystal water molecule. Evidence is presented for this oxygen-atom as the site of protonation in solution as well. In 5.0 M CF3SO3H a slow reaction of the carbonate complex, quantitatively yielding the [Co([3(5)]adz)(H2O)(2)](3+) ion, was observed. k(obs) 7.9(1) x 10(-6) s(-1) at 25 degreesC.