A Unique Mechanism for Base Catalyzed Hydrolysis of Pentaaminecobalt(III) Complexes Containing Picolyl Residues DickieAlistair J. HocklessDavid C. R. WillisAnthony C. McKeonJosephine A. JacksonW. Gregory 2003 A novel [Co(pentaamine)Cl]<sup>2+</sup> complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2‘-pyridyl)-7-methyl-1,4,7-triazacyclononane). This <i>asym</i>-[Co(dmpmetacn)Cl]<sup>2+</sup> species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl<sub>4</sub><sup>2-</sup> salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH]<sup>2+</sup> is base catalyzed (<i>k</i><sub>OH</sub> 0.70 M<sup>-1</sup> s<sup>-1</sup>, 25 °C, <i>I</i> = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two −CH<sub>2</sub>− pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of <i>asym</i>-[Co(dmpmetacn)Cl]<sup>2+</sup> and control experiments on H/D exchange for the product <i>asym</i>-[Co(dmpmetacn)OD]<sup>2+</sup> in OD<sup>-</sup> show that each act of deprotonation at the acidic methylene leads to loss of Cl<sup>-</sup>. This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.