Secondary Deuterium Kinetic Isotope Effect for Aquation, Solvolysis, and Isomerization Reactions of <i>trans</i>-[Co(en)<sub>2</sub>(OSMe<sub>2</sub>)N<sub>3</sub>]<sup>2+</sup>, and the Resolution of a Mechanistic Anomaly JacksonW. G. 2004 The two closely spaced NH signals in the <sup>1</sup>H NMR spectrum of <i>trans</i>-[Co(en)<sub>2</sub>(OSMe<sub>2</sub>)(N<sub>3</sub>)]<sup>2+</sup> have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co−N<sub>3</sub>) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND<sub>3</sub>(l) has been reinterpreted as “normal”, with inversion of the effective deprotonation site accompanying the act of substitution. The re-examination of this system required a repeat study of the secondary isotope effect for the acid hydrolysis reaction, previously used to assign syn and anti amine sites, and this has been extended to other solvents (Me<sub>2</sub>SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me<sub>2</sub>SO substitution is observed for deuterium incorporation into the <i>cis</i>-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me<sub>2</sub>SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.