Although at first glance, the old reaction of B2H6 (diborane) with NH3 seemed to be simple, it has been shown that the reaction was complex and had perplexed boron chemists for decades. The reaction was highly sensitive to the experimental conditions, and one or the mixture of the products, such as diammoniate of diborane (DADB, [NH3BH2NH3][BH4]), ammonia borane (AB, NH3·BH3), aminodiborane (ADB, NH2B2H5), could be obtained. It was known that the asymmetrical cleavage of the double bridge bond of B2H6 produced the ionic product DADB, whereas symmetrical cleavage afforded the covalent product, AB. The different cleavage patterns of B2H6 under particular conditions attracted interest to study the key points of the selectivity including steric effect, inductive effect, solvent effect, and dielectric constant, but it is still not possible to predict the products of the reactions of B2H6 with Lewis base. B2H6 was present in THF as the THF·BH3 adduct, and the products of the reactions of Lewis bases with THF·BH3or B2H6 were of the same types. The formation mechanisms of AB and DADB were explored by an ingenious design using THF·BH3 as a proxy for more reactive B2H6, and the reaction of THF·BH3 with NH3could be carried out under mild conditions. It was found that dihydrogen bonds played a critical role in the competing formation of AB and DADB. Both experimental and computational results demonstrated that dihydrogen bond was the dominant factor in the complex formation mechanism of ADB.