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The mechanism of the solvolysis/hydrolysis of ammonia borane by iridium (Ir), cobalt (Co), iron (Fe) and ruthenium (Ru) complexes with various PNP ligands has been revisited using density functional theory (DFT). The approach of ammonia borane (NH₃BH₃) to the metal center has been tested on three different possible mechanisms, namely, the stepwise, concerted and proton transfer mechanism. It was found that the theoretical analyses correlate with the experimental results very well, with the activities of the iridium complexes with different PNP ligands following the order: (tBu)₂P > (iPr)₂P > (Ph)₂P through the concerted mechanism. The reaction barriers of the rate-determining steps for the dehydrogenation of ammonia borane catalyzed by the active species [(^tBu)₂PNP-IrH] (Complex I-8), are found to be 19.3 kcal/mol (stepwise), 15.2 kcal/mol (concerted) and 26.8 kcal/mol (proton transfer), respectively. Thus, the concerted mechanism is the more kinetically favorable pathway. It is interesting to find that stable (^tBu)₂PNP Co-H₂O and (^tBu)₂PNP Co-NH₃ chelation products exist, which could stabilize the active I-8 species during the hydrolysis reaction cycle. The use of more sterically hindered and electron-donating PNP ligands such as (adamantyl)₂P- provides similar activity as the t-butyl analogue. This research provides insights into the design of efficient cobalt catalysts instead of using precious and noble metal, which could benefit the development of a more sustainable hydrogen economy. Full article