The mechanism of formation of ammonia borane (NH(3)BH(3), AB) and the diammoniate of diborane ([H(2)B(NH(3))(2)][BH(4)], DADB) in the reaction between NH(3) and THF·BH(3) was explored experimentally and computationally. Ammonia diborane (NH(3)BH(2)(μ-H)BH(3), AaDB), a long-sought intermediate proposed for the formation of DADB, was directly observed in the reaction using (11)B NMR spectroscopy. The results indicate that dihydrogen bonds between the initially formed AB and AaDB accelerate the formation of DADB in competition with the formation of AB.