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.