The [Co(dmptacn)Cl](2+) (dmptacn = 1,4-bis(pyridylmethyl)-1,4,7-triazacyclononane) complex has been shown to be the asym isomer through 1D and 2D NMR studies, its optical resolution, and the single-crystal X-ray structure of its perchlorate salt. The kinetics of base-catalyzed hydrolysis establishes the usual [OH(-)] dependence (k(OH) = 0.040 M(-1) s(-1), 25 degrees C, I = 1.0 M, NaCl), but D-exchange experiments reveal that substantial if not complete reaction proceeds via the new pseudoaminate mechanism, i.e., via deprotonation at an alpha-CH(2) center rather than the NH. The significant kinetic isotope effect (k(H)/k(D) = 2.1) is interpreted in terms of rate-limiting deprotonation followed by reprotonation of the conjugate base at a rate competitive with loss of Cl(-). NMR and polarimetric studies establish geometric and optical retention for the hydrolysis reaction and exclude even the transient formation of a sym isomer intermediate.