The water exchange reaction on Co(NH3)(5)OH23+ was investigated with various density functionals and basis sets. A Gibbs activation energy (Delta G(double dagger)) agreeing with experiment was obtained with the long-range-corrected functionals omega B97X-D3 and LC-BOP-LRD, SMD hydration, and modified Karlsruhe def2-TZVP basis sets. This computational technique was then applied to the reaction of NO+ with Co(NH3)(5)N-3(2+). All of the possible pathways were investigated, NO+ attack at the terminal N of Co(NH3)(5)N-3(2+) via the E and the Z isomers of the transition states, and NO+ attack at the bound N of azide, also via both isomers. The most favorable pathway proceeds via the attack at the bound N via the Z isomer. This leads to the intermediate with an oxatetrazole ligand bound to Co (III) at the N in the 3-position, Co(NH3)(5)(cycl-N4O)(3+), which undergoes N-2 elimination to yield the Co(NH3)(5)NN2O3+ intermediate. The subsequent substitution of N2O by water follows the Id mechanism with retention of the configuration. No evidence for the existence of the square-pyramidal pentacoordinated intermediate Co(NH3)(5)(3+) was found. All of the investigated intermediates, Co(NH3)(5)N-2(3+), Co(NH3)(5)[E-N(N-2) (NO)](3+), CO (NH3)(5)(E-ON4)(3+), CO(NH3)(5)ON23+, CO(NH3)(5)(cycl-N4O)(3+), and Co(NH3)(5)N2O3+, exhibit short lifetimes of less than similar to 60 mu s and react via the I-d mechanism.