Secondary Deuterium Kinetic Isotope Effect for Aquation, Solvolysis, and Isomerization Reactions of trans-[Co(en)₂(OSMe₂)N₃]²⁺, and the Resolution of a Mechanistic Anomaly

The two closely spaced NH signals in the ¹H NMR spectrum of trans-[Co(en)₂(OSMe₂)(N₃)]²⁺ have been reassigned using 2D NMR and other techniques. Thus, the unusual syn to anti (to Co−N₃) NH rearrangement on base catalyzed substitution of the selectively deuterated complex in ND₃(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 (Me₂SO, MeCN). The relative NH proton exchange rates are also reconsidered. A systematic rate reduction for Me₂SO substitution is observed for deuterium incorporation into the cis-NH centers, irrespective of whether these are syn or anti, and the effect is much greater in Me₂SO than in water. The results are interpreted in terms of zero point energy effects and coupled vibrations.