The study of ligand substitution reactions of transition metal complexes is often a significant component of undergraduate courses in inorganic chemistry. In the laboratory, many undergraduates have studied the aquation of [Co(NH3)5Cl]2+at elevated temperatures, taking aliquots of the reaction mixture, and obtaining a series of UV–vis spectra. Recently, an improved synthesis of a similar complex, trans-[Co(NH3)4Cl2]+ was reported. This complex undergoes aquation in less than an hour at room temperature, much faster than [Co(NH3)5Cl]2+, and is readily monitored by 1H NMR spectroscopy. This reaction forms the basis of an instructive, multi-week laboratory experience in inorganic synthesis, followed by an analysis of a reaction mechanism using a classical activation parameter determination.