Though numerous catalysts for the dehydrogenation of ammonia borane (AB) are known, those that release >2 equiv of H_2 are uncommon. Herein, we report the synthesis of Mo complexes supported by a para-terphenyl diphosphine ligand, 1, displaying metal–arene interactions. Both a Mo^0 N_2 complex, 5, and a Mo^(II) bis(acetonitrile) complex, 4, exhibit high levels of AB dehydrogenation, releasing over 2.0 equiv of H_2. The reaction rate, extent of dehydrogenation, and reaction mechanism vary as a function of the precatalyst oxidation state. Several Mo hydrides (Mo^(II)(H)_2, [Mo^(II)(H)]^+, and [Mo^(IV)(H)_3]^+) relevant to AB chemistry were characterized.
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