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